U.S. patent number 10,716,148 [Application Number 16/065,766] was granted by the patent office on 2020-07-14 for method and user equipment for transmitting random access preamble.
This patent grant is currently assigned to LG Electronics Inc.. The grantee listed for this patent is LG Electronics Inc.. Invention is credited to Byounghoon Kim, Eunsun Kim, Kijun Kim, Hyunsoo Ko, Sukhyon Yoon.
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United States Patent |
10,716,148 |
Kim , et al. |
July 14, 2020 |
Method and user equipment for transmitting random access
preamble
Abstract
In the present invention, when a UE transmits a random access
preamble, the UE maintains a power ramping counter used for
determination of a transmission power equally to a value of
previous transmission without incrementing the power ramping
counter if a target synchronization signal block is changed
differently from a previous random access preamble
transmission.
Inventors: |
Kim; Eunsun (Seoul,
KR), Kim; Kijun (Seoul, KR), Kim;
Byounghoon (Seoul, KR), Yoon; Sukhyon (Seoul,
KR), Ko; Hyunsoo (Seoul, KR) |
Applicant: |
Name |
City |
State |
Country |
Type |
LG Electronics Inc. |
Seoul |
N/A |
KR |
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Assignee: |
LG Electronics Inc. (Seoul,
KR)
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Family
ID: |
63447783 |
Appl.
No.: |
16/065,766 |
Filed: |
March 7, 2018 |
PCT
Filed: |
March 07, 2018 |
PCT No.: |
PCT/KR2018/002705 |
371(c)(1),(2),(4) Date: |
December 19, 2018 |
PCT
Pub. No.: |
WO2018/164478 |
PCT
Pub. Date: |
September 13, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190394805 A1 |
Dec 26, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62548406 |
Aug 22, 2017 |
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62544079 |
Aug 11, 2017 |
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62524607 |
Jun 25, 2017 |
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62521533 |
Jun 19, 2017 |
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62516062 |
Jun 6, 2017 |
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62511359 |
May 26, 2017 |
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62468257 |
Mar 7, 2017 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04W
74/08 (20130101); H04W 52/36 (20130101); H04W
52/42 (20130101); H04W 74/0833 (20130101); H04W
56/00 (20130101); H04W 52/50 (20130101); H04W
52/38 (20130101) |
Current International
Class: |
H04W
74/08 (20090101); H04W 52/42 (20090101); H04W
52/36 (20090101); H04W 52/50 (20090101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3021621 |
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May 2016 |
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EP |
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WO2013112646 |
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Aug 2013 |
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WO |
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Other References
Zte et al., "Design of SS Burst Set and SS Block Index",
R1-1701573, 3GPP TSG RAN WGI Meeting #88, Athens, Greece, Feb. 7,
2017, See sections 1-2. cited by applicant .
International Search Report and Written Opinion in International
Application No. PCT/KR2018/002705, dated Jun. 22, 2018, 18 pages.
cited by applicant .
Zte et al., "WF on NR Rach Msg. 1 Re-Transmission," R1-1701261,
3GPP TSG RAN WG1 NR Ad Hoc, Spokane, Washington, USA, dated Jan.
16-20, 2017, 3 pages. cited by applicant .
Samsung, "4-step random access procedure," R1-1702909, 3GPP TSG RAN
WG1 Meeting #88, Athens, Greece, Feb. 13-17, 2017, 10 pages. cited
by applicant .
NTT Docomo, Inc, "Discussion on 4-step random access procedure for
NR," R1-1702831, 3GPP TSG RAN WG1 Meeting #88, Athens Greece, dated
Feb. 13-17, 2017, 10 pages. cited by applicant .
Sharp, "RACH procedure for multi-Tx beam operation," R1-1703235,
3GPP TSG RAN WG1 Meeting #88, Athens, Greece, Feb. 13-17, 2017, 4
pages. cited by applicant .
Mitsubishi Electric, "On RACH retransmission," R1-1700304, 3GPP
TSG-RAN WG1 NR adhoc, Spokane, Washington, USA, Jan. 16-20, 2017, 4
pages. cited by applicant .
Sony, "Considerations for NR 4-step RACH procedure," R1-1703129,
3GPP TSG RAN WG1 Meeting #88, Athens, Greece, Feb. 13-17, 2017, 4
pages. cited by applicant .
3rd Generation Partnership Project; Technical Specification Group
Radio Access Network; "Evolved Universal Terrestrial Radio Access
(E-UTRA); Medium Access Control (MAC) protocol specification
(Release 14)," 3GPP TS 36.321 V14.1.0, dated Dec. 2016, 98 pages,
XP55536997. cited by applicant .
CATT, "Further details on NR 4-step RA Procedure," R1-1702066, 3GPP
TSG RAN WG1 Meeting #88, Athens, Greece, dated Feb. 13-17, 2017, 6
pages, XP051209227. cited by applicant .
Extended European Search Report in European Application No.
18763831.7, dated Dec. 20, 2019, 12 pages. cited by applicant .
LG Electronics, "Discussion on RACH Procedure," R1-1702442, 3GPP
TSG RAN WG1 Meeting #88, Athens, Greece, dated Feb. 13-17, 2017, 4
pages, XP051209596. cited by applicant .
Samsung, "NR 4-step random access procedure," R1-1700891, 3GPP TSG
RAN WG1 NR Ad Hoc, Spokane, Washington, USA, dated Jan. 16-20,
2017, 14 pages, XP051208407. cited by applicant.
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Primary Examiner: Marcelo; Melvin C
Attorney, Agent or Firm: Fish & Richardson P.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a National Stage application under 35 U.S.C.
.sctn. 371 of International Application No. PCT/KR2018/002705,
filed on Mar. 7, 2018, which claims the benefit of U.S. Provisional
Application No. 62/548,406, filed on Aug. 22, 2017, U.S.
Provisional Application No. 62/544,079, filed on Aug. 11, 2017,
U.S. Provisional Application No. 62/524,607, filed on Jun. 25,
2017, U.S. Provisional Application No. 62/521,533, filed on Jun.
19, 2017, U.S. Provisional Application No. 62/516,062, filed on
Jun. 6, 2017, U.S. Provisional Application No. 62/511,359, filed on
May 26, 2017, and U.S. Provisional Application No. 62/468,257,
filed on Mar. 7, 2017. The disclosures of the prior applications
are incorporated by reference in their entirety.
Claims
The invention claimed is:
1. A method for transmitting, by a user equipment (UE), a random
access preamble in a wireless communication system, the method
comprising: performing a first random access preamble transmission
for a first synchronization signal (SS) block at a first
transmission power; and performing a second random access preamble
transmission for a second SS block at a second transmission power
based on not successfully receiving a random access response to the
first random access preamble transmission, wherein the second
transmission power is determined based on a power ramping counter
value used for determination of the first transmission power based
on the second SS block being different from the first SS block.
2. The method according to claim 1, wherein, based on the second SS
block being the same as the first SS block, the second transmission
power is determined: based on a power ramping counter value
incremented by 1 from the power ramping counter value used for
determination of the first transmission power when a transmission
(Tx) beam used for the second random access preamble transmission
is the same as a Tx beam used for the first random access preamble
transmission, and based on the same power ramping counter value as
that used for determination of the first transmission power when
the Tx beam used for the second random access preamble transmission
is different from the Tx beam used for the first random access
preamble transmission.
3. The method according to claim 1, wherein the first random access
preamble transmission is performed on a first random access channel
(RACH) resource associated with the first SS block, and wherein the
second random access preamble transmission is performed on a second
RACH resource associated with the second SS block.
4. The method according to claim 3, wherein the first RACH resource
is different from the second RACH resource based on the first SS
block being different from the second SS block.
5. The method according to claim 1, further comprising:
incrementing a preamble transmission counter by 1 to set the
preamble transmission counter to a first value for the first random
access preamble transmission; and setting the preamble transmission
counter to a second value by adding 1 to the first value for the
second random access preamble transmission.
6. The method according to claim 5, wherein the second random
access preamble transmission is performed only in a state in which
the second value does not exceed a maximum number of preamble
transmissions.
7. A user equipment (UE) configured to transmit a random access
preamble in a wireless communication system, the UE comprising: a
radio frequency (RF) transceiver; a processor; and a memory that is
connectable to the processor and that stores thereon at least one
computer program which, when executed, causes the processor to
perform operations comprising: performing a first random access
preamble transmission for a first synchronization signal (SS) block
at a first transmission power; and performing a second random
access preamble transmission for a second SS block at a second
transmission power based on not successfully receiving a random
access response to the first random access preamble transmission,
wherein the operations further comprise: determining the second
transmission power based on a power ramping counter value used for
determination of the first transmission power based on the second
SS block being different from the first SS block.
8. The UE according to claim 7, wherein, based on the second SS
block being the same as the first SS block, determining the second
transmission power comprises: determining the second transmission
power based on a power ramping counter value incremented by 1 from
the power ramping counter value used for determination of the first
transmission power when a transmission (Tx) beam used for the
second random access preamble transmission is the same as a Tx beam
used for the first random access preamble transmission, and
determining the second transmission power based on the same power
ramping counter value as that used for determination of the first
transmission power when the Tx beam used for the second random
access preamble transmission is different from the Tx beam used for
the first random access preamble transmission.
9. The UE according to claim 7, wherein the first random access
preamble transmission is performed on a first random access channel
(RACH) resource associated with the first SS block, and the second
random access preamble transmission is performed on a second RACH
resource associated with the second SS block.
10. The UE according to claim 9, wherein the first RACH resource is
different from the second RACH resource based on the first SS block
being different from the second SS block.
11. The UE according to claim 7, wherein the operations further
comprise: incrementing a preamble transmission counter by 1 to set
the preamble transmission counter to a first value for the first
random access preamble transmission, and setting the preamble
transmission counter to a second value by adding 1 to the first
value for the second random access preamble transmission.
12. The UE according to claim 11, wherein the second random access
preamble transmission is performed only in a state in which the
second value does not exceed a maximum number of preamble
transmissions.
13. A processing apparatus configured to control a user equipment
(UE) to transmit a random access preamble in a wireless
communication system, the processing apparatus comprising: a
processor; and a memory that is connectable to the processor and
that stores thereon at least one computer program which, when
executed, causes the processor to perform operations comprising:
performing a first random access preamble transmission for a first
synchronization signal (SS) block at a first transmission power;
and performing a second random access preamble transmission for a
second SS block at a second transmission power based on not
successfully receiving a random access response to the first random
access preamble transmission, wherein the operations further
comprise: determining the second transmission power based on a
power ramping counter value used for determination of the first
transmission power based on the second SS block being different
from the first SS block.
14. The processing apparatus according to claim 13, wherein, based
on the second SS block being the same as the first SS block,
determining the second transmission power comprises: determining
the second transmission power based on a power ramping counter
value incremented by 1 from the power ramping counter value used
for determination of the first transmission power when a
transmission (Tx) beam used for the second random access preamble
transmission is the same as a Tx beam used for the first random
access preamble transmission, and determining the second
transmission power based on the same power ramping counter value as
that used for determination of the first transmission power when
the Tx beam used for the second random access preamble transmission
is different from the Tx beam used for the first random access
preamble transmission.
15. The processing apparatus according to claim 13, wherein the
first random access preamble transmission is performed on a first
random access channel (RACH) resource associated with the first SS
block, and the second random access preamble transmission is
performed on a second RACH resource associated with the second SS
block.
16. The processing apparatus according to claim 15, wherein the
first RACH resource is different from the second RACH resource
based on the first SS block being different from the second SS
block.
17. The processing apparatus according to claim 13, wherein the
operations further comprise: incrementing a preamble transmission
counter by 1 to set the preamble transmission counter to a first
value for the first random access preamble transmission, and
setting the preamble transmission counter to a second value by
adding 1 to the first value for the second random access preamble
transmission.
18. The processing apparatus according to claim 17, wherein the
second random access preamble transmission is performed only in a
state in which the second value does not exceed a maximum number of
preamble transmissions.
Description
TECHNICAL FIELD
The present invention relates to a wireless communication system.
More particularly, the present invention relates to a method and
apparatus for transmitting a random access preamble.
BACKGROUND ART
With appearance and spread of machine-to-machine (M2M)
communication and a variety of devices such as smartphones and
tablet PCs and technology demanding a large amount of data
transmission, data throughput needed in a cellular network has
rapidly increased. To satisfy such rapidly increasing data
throughput, carrier aggregation technology, cognitive radio
technology, etc. for efficiently employing more frequency bands and
multiple input multiple output (MIMO) technology, multi-base
station (BS) cooperation technology, etc. for raising data capacity
transmitted on limited frequency resources have been developed.
A general wireless communication system performs data
transmission/reception through one downlink (DL) band and through
one uplink (UL) band corresponding to the DL band (in case of a
frequency division duplex (FDD) mode), or divides a prescribed
radio frame into a UL time unit and a DL time unit in the time
domain and then performs data transmission/reception through the
UL/DL time unit (in case of a time division duplex (TDD) mode). A
base station (BS) and a user equipment (UE) transmit and receive
data and/or control information scheduled on a prescribed time unit
basis, e.g. on a subframe basis. The data is transmitted and
received through a data region configured in a UL/DL subframe and
the control information is transmitted and received through a
control region configured in the UL/DL subframe. To this end,
various physical channels carrying radio signals are formed in the
UL/DL subframe. In contrast, carrier aggregation technology serves
to use a wider UL/DL bandwidth by aggregating a plurality of UL/DL
frequency blocks in order to use a broader frequency band so that
more signals relative to signals when a single carrier is used can
be simultaneously processed.
In addition, a communication environment has evolved into
increasing density of nodes accessible by a user at the periphery
of the nodes. A node refers to a fixed point capable of
transmitting/receiving a radio signal to/from the UE through one or
more antennas. A communication system including high-density nodes
may provide a better communication service to the UE through
cooperation between the nodes.
As more communication devices have demanded higher communication
capacity, there has been necessity of enhanced mobile broadband
(eMBB) relative to legacy radio access technology (RAT). In
addition, massive machine type communication (mMTC) for providing
various services anytime and anywhere by connecting a plurality of
devices and objects to each other is one main issue to be
considered in future-generation communication.
Further, a communication system to be designed in consideration of
services/UEs sensitive to reliability and latency is under
discussion. The introduction of future-generation RAT has been
discussed by taking into consideration eMBB communication, mMTC,
ultra-reliable and low-latency communication (URLLC), and the
like.
DISCLOSURE
Technical Problem
Due to introduction of new radio communication technology, the
number of user equipments (UEs) to which a BS should provide a
service in a prescribed resource region increases and the amount of
data and control information that the BS should transmit to the UEs
increases. Since the amount of resources available to the BS for
communication with the UE(s) is limited, a new method in which the
BS efficiently receives/transmits uplink/downlink data and/or
uplink/downlink control information using the limited radio
resources is needed.
With development of technologies, overcoming delay or latency has
become an important challenge. Applications whose performance
critically depends on delay/latency are increasing. Accordingly, a
method to reduce delay/latency compared to the legacy system is
demanded.
Also, with development of smart devices, a new scheme for
efficiently transmitting/receiving a small amount of data or
efficiently transmitting/receiving data occurring at a low
frequency is required.
In addition, a signal transmission/reception method is required in
the system supporting new radio access technologies using high
frequency bands.
The technical objects that can be achieved through the present
invention are not limited to what has been particularly described
hereinabove and other technical objects not described herein will
be more clearly understood by persons skilled in the art from the
following detailed description.
Technical Solution
In one aspect of the present invention, a method for transmitting a
random access preamble from a user equipment (UE) in a wireless
communication system is provided. The method comprises: performing
a first random access preamble transmission for a first
synchronization signal (SS) block at a first transmission power;
and performing a second random access preamble transmission for a
second SS block at a second transmission power if a random access
response to the first random access preamble transmission is not
received successfully. The second transmission power is determined
based on a power ramping counter value used for determination of
the first transmission power if the second SS block is different
from the first SS block.
In another aspect of the present invention, a UE for transmitting a
random access preamble in a wireless communication system is
provided. The UE comprises a radio frequency (RF) unit; and a
processor configured to control the RF unit. The processor is
configured to: perform a first random access preamble transmission
for a first synchronization signal (SS) block at a first
transmission power, and perform a second random access preamble
transmission for a second SS block at a second transmission power
if a random access response to the first random access preamble
transmission is not received successfully. The processor is
configured to determine the second transmission power based on a
power ramping counter value used for determination of the first
transmission power if the second SS block is different from the
first SS block.
In each aspect of the present invention, when the second SS block
is equal to the first SS block, the second transmission power may
be determined based on a power ramping counter value increased as
much as 1 from the power ramping counter value used for
determination of the first transmission power if a transmission
(Tx) beam used for the second random access preamble transmission
is equal to a Tx beam used for the first random access preamble
transmission.
In each aspect of the present invention, when the second SS block
is equal to the first SS block, the second transmission power may
be determined based on a power ramping counter value equal to the
power ramping counter value used for determination of the first
transmission power if the Tx beam used for the second random access
preamble transmission is different from Tx beam used for the first
random access preamble transmission.
In each aspect of the present invention, the first random access
preamble transmission may be performed using a first random access
channel (RACH) resource associated with the first SS block. The
second random access preamble transmission may be performed using a
second RACH resource associated with the second SS block.
In each aspect of the present invention, the first RACH resource
may be different from the second RACH resource if the first SS
block is different from the second SS block.
In each aspect of the present invention, a preamble transmission
counter for the first random access preamble transmission may be
set to a first value by increasing the preamble transmission
counter as much as 1. The preamble transmission counter may be set
to a second value by adding 1 to the first value for the second
random access preamble transmission.
In each aspect of the present invention, the first random access
preamble transmission may be performed only if the first value does
not exceed the maximum number of preamble transmissions.
In each aspect of the present invention, the second random access
preamble transmission may be performed only if the second value
does not exceed the maximum number of preamble transmissions.
The above technical solutions are merely some parts of the
embodiments of the present invention and various embodiments into
which the technical features of the present invention are
incorporated can be derived and understood by persons skilled in
the art from the following detailed description of the present
invention.
Advantageous Effects
According to the present invention, uplink/downlink signals can be
efficiently transmitted/received. Therefore, overall throughput of
a radio communication system can be improved.
According to an embodiment of the present invention, delay/latency
occurring during communication between a user equipment and a base
station may be reduced.
In addition, owing to development of smart devices, it is possible
to efficiently transmit/receive not only a small amount of data but
also data which occurs infrequently.
Moreover, signals can be transmitted/received in the system
supporting new radio access technologies.
It will be appreciated by persons skilled in the art that that the
effects that can be achieved through the present invention are not
limited to what has been particularly described hereinabove and
other advantages of the present invention will be more clearly
understood from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further
understanding of the invention, illustrate embodiments of the
invention and together with the description serve to explain the
principle of the invention.
FIG. 1 illustrates a random access procedure in an LTE/LTE-A
system.
FIG. 2 illustrates a random access preamble format in a legacy
LTE/LTE-A system.
FIG. 3 illustrates a subframe structure available in a new radio
access technology (NR).
FIG. 4 abstractly illustrates transceiver units (TXRUs) and a
hybrid beamforming structure in terms of physical antennas.
FIG. 5 illustrates a cell of a new radio access technology (NR)
system.
FIG. 6 illustrates problems that may occur when a UE maintains a
power ramping counter while changing Tx beams for transmitting RACH
preamble.
FIG. 7 illustrates a beam switching method for RACH preamble
transmission/retransmission.
FIGS. 8 and 9 illustrate PRACH transmission/retransmission and
corresponding PRACH transmission power according to the present
invention.
FIG. 10 is a block diagram illustrating elements of a transmitting
device 10 and a receiving device 20 for implementing the present
invention.
FIG. 11 illustrates an example of a method for determining a PRACH
transmission power.
MODE FOR CARRYING OUT THE INVENTION
Reference will now be made in detail to the exemplary embodiments
of the present invention, examples of which are illustrated in the
accompanying drawings. The detailed description, which will be
given below with reference to the accompanying drawings, is
intended to explain exemplary embodiments of the present invention,
rather than to show the only embodiments that can be implemented
according to the invention. The following detailed description
includes specific details in order to provide a thorough
understanding of the present invention. However, it will be
apparent to those skilled in the art that the present invention may
be practiced without such specific details.
In some instances, known structures and devices are omitted or are
shown in block diagram form, focusing on important features of the
structures and devices, so as not to obscure the concept of the
present invention. The same reference numbers will be used
throughout this specification to refer to the same or like
parts.
The following techniques, apparatuses, and systems may be applied
to a variety of wireless multiple access systems. Examples of the
multiple access systems include a code division multiple access
(CDMA) system, a frequency division multiple access (FDMA) system,
a time division multiple access (TDMA) system, an orthogonal
frequency division multiple access (OFDMA) system, a single carrier
frequency division multiple access (SC-FDMA) system, and a
multicarrier frequency division multiple access (MC-FDMA) system.
CDMA may be embodied through radio technology such as universal
terrestrial radio access (UTRA) or CDMA2000. TDMA may be embodied
through radio technology such as global system for mobile
communications (GSM), general packet radio service (GPRS), or
enhanced data rates for GSM evolution (EDGE). OFDMA may be embodied
through radio technology such as institute of electrical and
electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),
IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is a part of a
universal mobile telecommunications system (UMTS). 3rd generation
partnership project (3GPP) long term evolution (LTE) is a part of
evolved UMTS (E-UMTS) using E-UTRA. 3GPP LTE employs OFDMA in DL
and SC-FDMA in UL. LTE-advanced (LTE-A) is an evolved version of
3GPP LTE. For convenience of description, it is assumed that the
present invention is applied to 3GPP based communication system,
e.g. LTE/LTE-A, NR. However, the technical features of the present
invention are not limited thereto. For example, although the
following detailed description is given based on a mobile
communication system corresponding to a 3GPP LTE/LTE-A/NR system,
aspects of the present invention that are not specific to 3GPP
LTE/LTE-A/NR are applicable to other mobile communication
systems.
For example, the present invention is applicable to contention
based communication such as Wi-Fi as well as non-contention based
communication as in the 3GPP LTE/LTE-A system in which an eNB
allocates a DL/UL time/frequency resource to a UE and the UE
receives a DL signal and transmits a UL signal according to
resource allocation of the eNB. In a non-contention based
communication scheme, an access point (AP) or a control node for
controlling the AP allocates a resource for communication between
the UE and the AP, whereas, in a contention based communication
scheme, a communication resource is occupied through contention
between UEs which desire to access the AP. The contention based
communication scheme will now be described in brief. One type of
the contention based communication scheme is carrier sense multiple
access (CSMA). CSMA refers to a probabilistic media access control
(MAC) protocol for confirming, before a node or a communication
device transmits traffic on a shared transmission medium (also
called a shared channel) such as a frequency band, that there is no
other traffic on the same shared transmission medium. In CSMA, a
transmitting device determines whether another transmission is
being performed before attempting to transmit traffic to a
receiving device. In other words, the transmitting device attempts
to detect presence of a carrier from another transmitting device
before attempting to perform transmission. Upon sensing the
carrier, the transmitting device waits for another transmission
device which is performing transmission to finish transmission,
before performing transmission thereof. Consequently, CSMA can be a
communication scheme based on the principle of "sense before
transmit" or "listen before talk". A scheme for avoiding collision
between transmitting devices in the contention based communication
system using CSMA includes carrier sense multiple access with
collision detection (CSMA/CD) and/or carrier sense multiple access
with collision avoidance (CSMA/CA). CSMA/CD is a collision
detection scheme in a wired local area network (LAN) environment.
In CSMA/CD, a personal computer (PC) or a server which desires to
perform communication in an Ethernet environment first confirms
whether communication occurs on a network and, if another device
carries data on the network, the PC or the server waits and then
transmits data. That is, when two or more users (e.g. PCs, UEs,
etc.) simultaneously transmit data, collision occurs between
simultaneous transmission and CSMA/CD is a scheme for flexibly
transmitting data by monitoring collision. A transmitting device
using CSMA/CD adjusts data transmission thereof by sensing data
transmission performed by another device using a specific rule.
CSMA/CA is a MAC protocol specified in IEEE 802.11 standards. A
wireless LAN (WLAN) system conforming to IEEE 802.11 standards does
not use CSMA/CD which has been used in IEEE 802.3 standards and
uses CA, i.e. a collision avoidance scheme. Transmission devices
always sense carrier of a network and, if the network is empty, the
transmission devices wait for determined time according to
locations thereof registered in a list and then transmit data.
Various methods are used to determine priority of the transmission
devices in the list and to reconfigure priority. In a system
according to some versions of IEEE 802.11 standards, collision may
occur and, in this case, a collision sensing procedure is
performed. A transmission device using CSMA/CA avoids collision
between data transmission thereof and data transmission of another
transmission device using a specific rule.
In embodiments of the present invention described below, the term
"assume" may mean that a subject to transmit a channel transmits
the channel in accordance with the corresponding "assumption". This
may also mean that a subject to receive the channel receives or
decodes the channel in a form conforming to the "assumption", on
the assumption that the channel has been transmitted according to
the "assumption".
In the present invention, puncturing a channel on a specific
resource means that the signal of the channel is mapped to the
specific resource in the procedure of resource mapping of the
channel, but a portion of the signal mapped to the punctured
resource is excluded in transmitting the channel. In other words,
the specific resource which is punctured is counted as a resource
for the channel in the procedure of resource mapping of the
channel, a signal mapped to the specific resource among the signals
of the channel is not actually transmitted. The receiver of the
channel receives, demodulates or decodes the channel, assuming that
the signal mapped to the specific resource is not transmitted. On
the other hand, rate-matching of a channel on a specific resource
means that the channel is never mapped to the specific resource in
the procedure of resource mapping of the channel, and thus the
specific resource is not used for transmission of the channel. In
other words, the rate-matched resource is not counted as a resource
for the channel in the procedure of resource mapping of the
channel. The receiver of the channel receives, demodulates, or
decodes the channel, assuming that the specific rate-matched
resource is not used for mapping and transmission of the
channel.
In the present invention, a user equipment (UE) may be a fixed or
mobile device. Examples of the UE include various devices that
transmit and receive user data and/or various kinds of control
information to and from a base station (BS). The UE may be referred
to as a terminal equipment (TE), a mobile station (MS), a mobile
terminal (MT), a user terminal (UT), a subscriber station (SS), a
wireless device, a personal digital assistant (PDA), a wireless
modem, a handheld device, etc. In addition, in the present
invention, a BS generally refers to a fixed station that performs
communication with a UE and/or another BS, and exchanges various
kinds of data and control information with the UE and another BS.
The BS may be referred to as an advanced base station (ABS), a
node-B (NB), an evolved node-B (eNB), a base transceiver system
(BTS), an access point (AP), a processing server (PS), etc.
Particularly, a BS of a UTRAN is referred to as a Node-B, a BS of
an E-UTRAN is referred to as an eNB, and a BS of a new radio access
technology network is referred to as a gNB. In describing the
present invention, a BS will be referred to as a gNB.
In the present invention, a node refers to a fixed point capable of
transmitting/receiving a radio signal through communication with a
UE. Various types of gNBs may be used as nodes irrespective of the
terms thereof. For example, a BS, a node B (NB), an e-node B (eNB),
a pico-cell eNB (PeNB), a home eNB (HeNB), gNB, a relay, a
repeater, etc. may be a node. In addition, the node may not be a
gNB. For example, the node may be a radio remote head (RRH) or a
radio remote unit (RRU). The RRH or RRU generally has a lower power
level than a power level of a gNB. Since the RRH or RRU
(hereinafter, RRH/RRU) is generally connected to the gNB through a
dedicated line such as an optical cable, cooperative communication
between RRH/RRU and the gNB can be smoothly performed in comparison
with cooperative communication between gNBs connected by a radio
line. At least one antenna is installed per node. The antenna may
mean a physical antenna or mean an antenna port or a virtual
antenna.
In the present invention, a cell refers to a prescribed
geographical area to which one or more nodes provide a
communication service. Accordingly, in the present invention,
communicating with a specific cell may mean communicating with a
gNB or a node which provides a communication service to the
specific cell. In addition, a DL/UL signal of a specific cell
refers to a DL/UL signal from/to a gNB or a node which provides a
communication service to the specific cell. A node providing UL/DL
communication services to a UE is called a serving node and a cell
to which UL/DL communication services are provided by the serving
node is especially called a serving cell. Furthermore, channel
status/quality of a specific cell refers to channel status/quality
of a channel or communication link formed between a gNB or node
which provides a communication service to the specific cell and a
UE. In the 3GPP based communication system, the UE may measure DL
channel state received from a specific node using cell-specific
reference signal(s) (CRS(s)) transmitted on a CRS resource and/or
channel state information reference signal(s) (CSI-RS(s))
transmitted on a CSI-RS resource, allocated by antenna port(s) of
the specific node to the specific node.
Meanwhile, a 3GPP based communication system uses the concept of a
cell in order to manage radio resources and a cell associated with
the radio resources is distinguished from a cell of a geographic
region.
A "cell" of a geographic region may be understood as coverage
within which a node can provide service using a carrier and a
"cell" of a radio resource is associated with bandwidth (BW) which
is a frequency range configured by the carrier. Since DL coverage,
which is a range within which the node is capable of transmitting a
valid signal, and UL coverage, which is a range within which the
node is capable of receiving the valid signal from the UE, depends
upon a carrier carrying the signal, the coverage of the node may be
associated with coverage of the "cell" of a radio resource used by
the node. Accordingly, the term "cell" may be used to indicate
service coverage of the node sometimes, a radio resource at other
times, or a range that a signal using a radio resource can reach
with valid strength at other times.
Meanwhile, the 3GPP communication standards use the concept of a
cell to manage radio resources. The "cell" associated with the
radio resources is defined by combination of downlink resources and
uplink resources, that is, combination of DL CC and UL CC. The cell
may be configured by downlink resources only, or may be configured
by downlink resources and uplink resources. If carrier aggregation
is supported, linkage between a carrier frequency of the downlink
resources (or DL CC) and a carrier frequency of the uplink
resources (or UL CC) may be indicated by system information. For
example, combination of the DL resources and the UL resources may
be indicated by linkage of system information block type 2 (51B2).
The carrier frequency means a center frequency of each cell or CC.
A cell operating on a primary frequency may be referred to as a
primary cell (Pcell) or PCC, and a cell operating on a secondary
frequency may be referred to as a secondary cell (Scell) or SCC.
The carrier corresponding to the Pcell on downlink will be referred
to as a downlink primary CC (DL PCC), and the carrier corresponding
to the Pcell on uplink will be referred to as an uplink primary CC
(UL PCC). A Scell means a cell that may be configured after
completion of radio resource control (RRC) connection establishment
and used to provide additional radio resources. The Scell may form
a set of serving cells for the UE together with the Pcell in
accordance with capabilities of the UE. The carrier corresponding
to the Scell on the downlink will be referred to as downlink
secondary CC (DL SCC), and the carrier corresponding to the Scell
on the uplink will be referred to as uplink secondary CC (UL SCC).
Although the UE is in RRC-CONNECTED state, if it is not configured
by carrier aggregation or does not support carrier aggregation, a
single serving cell configured by the Pcell only exists.
3GPP based communication standards define DL physical channels
corresponding to resource elements carrying information derived
from a higher layer and DL physical signals corresponding to
resource elements which are used by a physical layer but which do
not carry information derived from a higher layer. For example, a
physical downlink shared channel (PDSCH), a physical broadcast
channel (PBCH), a physical multicast channel (PMCH), a physical
control format indicator channel (PCFICH), a physical downlink
control channel (PDCCH), and a physical hybrid ARQ indicator
channel (PHICH) are defined as the DL physical channels, and a
reference signal and a synchronization signal are defined as the DL
physical signals. A reference signal (RS), also called a pilot,
refers to a special waveform of a predefined signal known to both a
BS and a UE. For example, a cell-specific RS (CRS), a UE-specific
RS (UE-RS), a positioning RS (PRS), and channel state information
RS (CSI-RS) may be defined as DL RSs. Meanwhile, the 3GPP LTE/LTE-A
standards define UL physical channels corresponding to resource
elements carrying information derived from a higher layer and UL
physical signals corresponding to resource elements which are used
by a physical layer but which do not carry information derived from
a higher layer. For example, a physical uplink shared channel
(PUSCH), a physical uplink control channel (PUCCH), and a physical
random access channel (PRACH) are defined as the UL physical
channels, and a demodulation reference signal (DM RS) for a UL
control/data signal and a sounding reference signal (SRS) used for
UL channel measurement are defined as the UL physical signals.
In the present invention, a physical downlink control channel
(PDCCH), a physical control format indicator channel (PCFICH), a
physical hybrid automatic retransmit request indicator channel
(PHICH), and a physical downlink shared channel (PDSCH) refer to a
set of time-frequency resources or resource elements (REs) carrying
downlink control information (DCI), a set of time-frequency
resources or REs carrying a control format indicator (CFI), a set
of time-frequency resources or REs carrying downlink
acknowledgement (ACK)/negative ACK (NACK), and a set of
time-frequency resources or REs carrying downlink data,
respectively. In addition, a physical uplink control channel
(PUCCH), a physical uplink shared channel (PUSCH) and a physical
random access channel (PRACH) refer to a set of time-frequency
resources or REs carrying uplink control information (UCI), a set
of time-frequency resources or REs carrying uplink data and a set
of time-frequency resources or REs carrying random access signals,
respectively. In the present invention, in particular, a
time-frequency resource or RE that is assigned to or belongs to
PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH is referred to as
PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH RE or
PDCCH/PCFICH/PHICH/PDSCH/PUCCH/PUSCH/PRACH time-frequency resource,
respectively. Therefore, in the present invention,
PUCCH/PUSCH/PRACH transmission of a UE is conceptually identical to
UCI/uplink data/random access signal transmission on
PUSCH/PUCCH/PRACH, respectively. In addition,
PDCCH/PCFICH/PHICH/PDSCH transmission of a gNB is conceptually
identical to downlink data/DCI transmission on
PDCCH/PCFICH/PHICH/PDSCH, respectively.
Hereinafter, OFDM symbol/subcarrier/RE to or for which
CRS/DMRS/CSI-RS/SRS/UE-RS/TRS is assigned or configured will be
referred to as CRS/DMRS/CSI-RS/SRS/UE-RS/TRS
symbol/carrier/subcarrier/RE. For example, an OFDM symbol to or for
which a tracking RS (TRS) is assigned or configured is referred to
as a TRS symbol, a subcarrier to or for which the TRS is assigned
or configured is referred to as a TRS subcarrier, and an RE to or
for which the TRS is assigned or configured is referred to as a TRS
RE. In addition, a subframe configured for transmission of the TRS
is referred to as a TRS subframe. Moreover, a subframe in which a
broadcast signal is transmitted is referred to as a broadcast
subframe or a PBCH subframe and a subframe in which a
synchronization signal (e.g. PSS and/or SSS) is transmitted is
referred to a synchronization signal subframe or a PSS/SSS
subframe. OFDM symbol/subcarrier/RE to or for which PSS/SSS is
assigned or configured is referred to as PSS/SSS
symbol/subcarrier/RE, respectively.
In the present invention, a CRS port, a UE-RS port, a CSI-RS port,
and a TRS port refer to an antenna port configured to transmit a
CRS, an antenna port configured to transmit a UE-RS, an antenna
port configured to transmit a CSI-RS, and an antenna port
configured to transmit a TRS, respectively. Antenna ports
configured to transmit CRSs may be distinguished from each other by
the locations of REs occupied by the CRSs according to CRS ports,
antenna ports configured to transmit UE-RSs may be distinguished
from each other by the locations of REs occupied by the UE-RSs
according to UE-RS ports, and antenna ports configured to transmit
CSI-RSs may be distinguished from each other by the locations of
REs occupied by the CSI-RSs according to CSI-RS ports. Therefore,
the term CRS/UE-RS/CSI-RS/TRS ports may also be used to indicate a
pattern of REs occupied by CRSs/UE-RSs/CSI-RSs/TRSs in a
predetermined resource region. In the present invention, both a
DMRS and a UE-RS refer to RSs for demodulation and, therefore, the
terms DMRS and UE-RS are used to refer to RSs for demodulation.
For terms and technologies which are not described in detail in the
present invention, reference can be made to the standard document
of 3GPP LTE/LTE-A, for example, 3GPP TS 36.211, 3GPP TS 36.212,
3GPP TS 36.213, 3GPP TS 36.321, and 3GPP TS 36.331 and the standard
document of 3GPP NR, for example, 3GPP TS 38.211, 3GPP TS 38.212,
3GPP 38.213, 3GPP 38.214, 3GPP 38.215, 3GPP TS 38.321, and 3GPP TS
36.331.
An operation to be first performed by the UE to receive services in
association with a specific system includes acquiring time and
frequency synchronization of the corresponding system, receiving
basic system information (SI), and synchronizing uplink timing to
an uplink. This procedure will be referred to as an initial access
procedure. The initial access procedure generally includes a
synchronization procedure and an RACH procedure (that is, random
access procedure). In an LTE/LTE-A system, when a UE is powered on
or desires to access a new cell, the UE perform an initial cell
search procedure including acquiring time and frequency
synchronization with the cell and detecting a physical layer cell
identity N.sup.cell.sub.ID of the cell. To this end, the UE may
receive synchronization signals, for example, a primary
synchronization signal (PSS) and a secondary synchronization signal
(SSS), from an eNB to thus establish synchronization with the eNB
and acquire information such as a cell identity (ID). For
convenience of description, the synchronization procedure in the
LTE/LTE-A system will briefly be described again. PSS: symbol
timing acquisition, frequency synchronization, and cell ID
detection within cell ID group (three hypotheses). SSS: cell ID
group detection (168 groups), 10 ms frame boundary detection, CP
detection (two types). PBCH decoding: antenna configuration, 40 ms
timing detection, system information, system bandwidth, etc.
That is, the UE acquires OFDM symbol timing and subframe timing
based on PSS and SSS and also acquires cell ID, and acquires
important information in the corresponding system by descrambling
and decoding a PBCH using a cell ID. After completing the
synchronization procedure, the UE performs the random access
procedure. In other words, after the initial cell search procedure,
the UE may perform a random access procedure to complete access to
the eNB. To this end, the UE may transmit a preamble through a
physical random access channel (PRACH) and receive a response
message to the preamble through a PDCCH and a PDSCH. After
performing the aforementioned procedures, the UE may perform
PDCCH/PDSCH reception and PUSCH/PUCCH transmission as a normal
UL/DL transmission procedure. The random access procedure is also
referred to as a random access channel (RACH) procedure. The random
access procedure is used for various purposes including initial
access, adjustment of UL synchronization, resource assignment, and
handover.
The random access procedure is classified into a contention-based
procedure and a dedicated (that is, non-contention-based)
procedure. The contention-based random access procedure is
generally used for initial access, and the dedicated random access
procedure is restrictively used for handover. In the
contention-based random access procedure, the UE randomly selects
RACH preamble sequence. Therefore, a plurality of UEs can transmit
the same RACH preamble sequence, whereby a contention resolution
procedure is required. On the other hand, in the dedicated random
access procedure, the UE uses RACH preamble sequence uniquely
allocated to a corresponding UE. Therefore, the UE may perform the
random access procedure without contention with another UE.
The contention-based random access procedure includes four steps as
follows. Hereinafter, messages transmitted in the steps 1 to 4 may
be referred to as 1 to 4 (Msg1 to Msg4). Step 1: RACH preamble (via
PRACH)(UE to eNB) Step 2: random access response (RAR)(via PDCCH
PDSCH)(eNB to UE) Step 3: layer 2/layer 3 message (via PUSCH)(UE to
eNB) Step 4: contention resolution message (eNB to UE)
The dedicated random access procedure includes three steps as
follows. Hereinafter, messages transmitted in steps 0 to 2 may be
referred to as messages 0 to 2 (Msg0 to Msg2). As a part of the
random access procedure, uplink transmission (that is, step 3)
corresponding to RAR may be performed. The dedicated random access
procedure may be triggered using a PDCCH (hereinafter, PDCCH order)
for commanding RACH preamble transmission. Step 0: RACH preamble
allocation (eNB to UE) through dedicated signaling Step 1: RACH
preamble (via PRACH)(UE to eNB) Step 2: random access response
(RAR)(via PDCCH PDSCH eNB to UE)
After transmitting the RACH preamble, the UE attempts to receive a
random access response (RAR) within a preset time window.
Specifically, the UE attempts to detect a PDCCH with a random
access radio network temporary identifier (RA-RNTI) (hereinafter,
RA-RNTI PDCCH) (e.g., CRC is masked with RA-RNTI on the PDCCH) in
the time window. In detecting the RA-RNTI PDCCH, the UE checks the
PDSCH corresponding to the RA-RNTI PDCCH for presence of an RAR
directed thereto. The RAR includes timing advance (TA) information
indicating timing offset information for UL synchronization, UL
resource allocation information (UL grant information), and a
temporary UE identifier (e.g., temporary cell-RNTI (TC-RNTI)). The
UE may perform UL transmission (of, e.g., Msg3) according to the
resource allocation information and the TA value in the RAR. HARQ
is applied to UL transmission corresponding to the RAR.
Accordingly, after transmitting Msg3, the UE may receive
acknowledgement information (e.g., PHICH) corresponding to
Msg3.
FIG. 1 illustrates a random access procedure in an LTE/LTE-A
system. RRC state is varied depending on RRC connection. The RRC
state means whether an entity of RRC layer of a UE is logically
connected with an entity of RRC layer of an eNB. The state that the
entity of the RRC layer of the UE is connected with the entity of
the RRC layer of the eNB means RRC connected state, and the state
that the entity of the RRC layer of the UE is not connected with
the entity of the RRC layer of the eNB means RRC idle state. The
presence of the UE of the idle state is identified in a unit of big
zone, and the UE should transition to a connected state to receive
a conventional mobile communication service such as voice or data.
When a user first turns on a power of the UE, the UE stays in the
idle mode in the corresponding cell after searching for a proper
cell. The UE stayed in the idle mode establishes an RRC connection
with the RRC layer of the eNB through an RRC connection procedure
when the RRC connection is required, and transitions to RRC
connected state. The RRC connection procedure includes a procedure
of transmitting an RRC connection request message from the UE to
the eNB, a procedure of transmitting an RRC connection setup
message from the eNB to the UE, and a procedure of transmitting an
RRC connection setup complete message from the UE to the eNB. Since
UL grant is required for transmission of the RRC connection request
message, the UE of the idle mode should perform a RACH procedure to
acquire UL grant. That is, the UE should transmit an RA preamble
(that is, Msg1) (S301) and receive an RAR (that is, Msg2) which is
a response to the RA preamble (S302). The UE transmits Msg3, which
includes RRC connection request message, to the eNB in accordance
with resource allocation information (that is, scheduling
information) and a timing advance value within the RAR (S303). If
the RRC connection request message is received from the UE, the eNB
accepts the RRC connection request of the UE if there are
sufficient radio resources, and transmits the RRC connection setup
message which is a response message to the UE (S304). If the UE
receives the RRC connection setup message, the UE transmits the RRC
connection setup complete message to the eNB (S305). If the UE
successfully transmits the RRC connection setup message, the UE
establishes an RRC connection with the eNB and transitions to the
RRC connection mode. That is, if the RACH procedure is completed,
the UE becomes the state that it is connected with the
corresponding cell.
FIG. 2 illustrates a random access preamble format in a legacy
LTE/LTE-A system.
In the legacy LTE/LTE-A system, a random access preamble, i.e., an
RACH preamble, includes a cyclic prefix having a length T.sub.CP
and a sequence part having a length T.sub.SEQ in a physical layer.
The parameter values T.sub.CP and T.sub.SEQ are listed in the
following table, and depend on the frame structure and the random
access configuration.
TABLE-US-00001 TABLE 1 Preamble format T.sub.CP T.sub.SEQ 0 3168
T.sub.s 24576 T.sub.s 1 21024 T.sub.s 24576 T.sub.s 2 6240 T.sub.s
2 24576 T.sub.s 3 21024 T.sub.s 2 24576 T.sub.s 4 448 T.sub.s 4096
T.sub.s
In the LTE/LTE-A system, the RACH preamble is transmitted in a UL
subframe. The transmission of a random access preamble is
restricted to certain time and frequency resources. These resources
are called PRACH resources, and enumerated in increasing order of
the subframe number within the radio frame and the PRBs in the
frequency domain such that index 0 correspond to the lowest
numbered PRB and subframe within the radio frame. Random access
resources are defined according to the PRACH configuration index
(refer to the standard document of 3GPP TS 36.211). The PRACH
configuration index is given by a higher layer signal (transmitted
by an eNB). In the LTE/LTE-A system, a subcarrier spacing .DELTA.f
is 15 kHz or 7.5 kHz. However, as given by Table 7, a subcarrier
spacing .DELTA.f.sub.RA for a random access preamble is 1.25 kHz or
0.75 kHz.
In case of a physical non-synchronized random access procedure in
the LTE/LTE-A system, the L1 random access procedure encompasses a
transmission of the random access preamble and a random access
response in view of the physical layer. The remaining messages are
scheduled for transmission by an upper layer on a common data
channel. The random access channel occupies 6 resource blocks
within one subframe or a set of consecutive subframes reserved for
random access preamble transmission. The eNB is not prohibited to
schedule data within the resource blocks reserved for random access
response. The following steps are required for layer 1 (L1) random
access procedure. Layer 1 procedure is triggered upon request of
preamble transmission by the higher layer. Preamble index, target
preamble received power PREAMBLE_RECEIVED_TARGET_POWER,
corresponding RA-RNTI and PRACH resource are indicated by the
higher layer as a part of the request. A preamble transmission
power P.sub.PRACH is determined as P.sub.PRACH=min{P.sub.CMAX,c(i),
PREAMBLE_RECEIVED_TARGET_POWER+PL.sub.c}_[dBm]. In this case,
P.sub.CMAX,c(i) is a configured UE transmission power for subframe
i of a service cell c, defined in 3GPP TS 36.101, and PL.sub.c is a
downlink path loss estimate value calculated for the serving cell c
within the UE. A preamble sequence is selected from a preamble
sequence set by using the preamble index. A single preamble is
transmitted using a selected preamble sequence at a transmission
power P.sub.PRACH on an indicated PRACH resource. Detection of
PDCCH is attempted with the indicated RA-RNTI during a window
controlled by the higher layer (see section 5.1.4 of 3GPP TS
36.321). If detected, a corresponding DL-SCH transport block is
passed to the higher layer. The higher layer parses the transport
block and indicates 20-bit uplink grant to the physical layer.
In case of the LTE/LTE-A system, a random access procedure in a
medium access control (MAC) layer is performed as follows: set
PREAMBLE_RECEIVED_TARGET_POWER
`preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(PREAMBLE_TRANSMISSION-
_COUNTER-1)*powerRampingStep`; if the UE is a bandwidth limited
(BL) UE or a UE within enforced coverage: the UE instructs the
physical layer to transmit a preamble with the number of
repetitions (that is, numRepetitionPerPreambleAttempt) required for
preamble transmission corresponding to a selected preamble group by
using a selected PRACH resource corresponding to a selected
enhanced coverage level, corresponding RA-RANTI, preamble index,
and PREAMBLE_RECEIVED_TARGET_POWER. else: and the UE instructs the
physical layer to transmit the preamble by using a selected PRACH,
corresponding RA-RNTI, preamble index and
PREAMBLE_RECEIVED_TARGET_POWER.
In the LTE/LTE-A system, information on UL transmission power for
RACH preamble transmission is also included in RACH configuration
and then delivered to the UE. For example,
preambleInitialReceivedTargetPower, powerRampingStep,
preambleTransMax, etc. are delivered to the UE by RRC signal as UE
common random access parameters (see PRACH-Config of 3GPP TS
36.331).
If the UE does not receive Msg2 within a certain time after
transmitting RACH Msg1 (that is, RACH preamble), that is, does not
receive RAR (that is, Msg2) within RAR window after transmitting
RACH Msg1 (that is, RACH preamble), the UE may retransmit RACH
Msg1. If the UE retransmits RACH Msg1, the UE may increase a
transmission power of the RACH Msg1 to be higher than a power
during previous transmission. In the LTE/LTE-A system, the
transmission power of the RACH Msg1 is increased as much as a power
ramping step by incrementing a layer-2 preamble transmission
counter of the UE by 1. PREAMBLE_TRANSMISSION_COUNTER starts from 1
and is incremented by 1 whenever preamble transmission is
attempted. If no RAR is received within RAR window, or if all the
received RARs do not include random access preamble identifier
corresponding to a random access preamble which was transmitted, it
is considered that RAR reception is not successful, and the UE
increments PREAMBLE_TRANSMISSION_COUNTER as much as 1. Preamble
transmission may be performed within the maximum number of preamble
transmissions preambleTransMax. For example, if
PREAMBLE_TRANSMISSION_COUNTER=preambleTransMax+1, the MAC layer
indicates a random access problem to the higher layer, and or
considers that the random access procedure is completed
unsuccessfully. DELTA_PREAMBLE is a value previously defined in
accordance with a preamble format as follows (see Table 7.6-1 of
3GPP TS 36.321).
TABLE-US-00002 TABLE 2 Preamble DELTA_PREAMBLE Format value 0 0 dB
1 0 dB 2 -3 dB 3 -3 dB 4 8 dB
In Table 2, a preamble format is given by prach-ConfigIndex (see
PRACH-Config of 3GPP TS 36.331).
In the current WiFi system, an unlicensed band which is not
dedicated for a specific operator is used for communication. On
this unlicensed band, if a certain reference, for example, a
technology for not causing interference in a radio channel or
minimizing interference is adopted, and if a certain output power
or less is used, all radio technologies may be used. Therefore,
there is the trend toward application of the technology currently
used in the cellular network to the unlicensed band. The trend is
referred to as licensed assisted access (LAA). Currently, as users
who use mobile data are increased explosively as compared with
frequencies (that is, licensed band(s)) owned by each radio
communication service operator, it is considered to introduce LAA
to the LTE system to enhance satisfaction of a user by providing
services in the unlicensed band. According to the LAA, a frequency
band which is not specified by 3GPP, that is, the unlicensed band
may be used for the LTE radio frequency. A WLAN band may be a main
application target of LAA. Basically, since radio transmission and
reception through contention between communication nodes is assumed
in the unlicensed band, channel sensing (CS) is performed before
each communication node transmits a signal, whereby it is required
to identify that another communication mode does not perform signal
transmission in the channel. This is referred to as clear channel
assessment (CCA), and the eNB or the UE of the LTE system should
perform CCA to transmit a signal in the unlicensed band
(hereinafter, referred to as LTE-U band). Also, when the eNB or the
UE of the LTE system transmits a signal, other communication nodes
such as WiFi should perform CCA so as not to cause interference.
For example, in the WiFi standard (e.g., 801.11ac), a CCA threshold
value is -62 dBm for a non-WiFi signal and -82 dBm for a WiFi
signal. This means that a station (STA) or an access point (AP)
does not perform signal transmission so as not to cause
interference if a signal other than WiFi is received at a power of
-62 dBm or more. Particularly, in the WiFi system, the STA or the
AP may perform CCA and signal transmission if a signal of a CCA
threshold value or more is not detected for 4 us or more.
Recently, as more communication devices (e.g. MTC devices, IoT
devices, and etc.) have demanded higher communication capacity,
there has been necessity of enhanced mobile broadband relative to
legacy radio access technology (RAT). In addition, massive machine
type communication for providing various services irrespective of
time and place by connecting a plurality of devices and objects to
each other is one main issue to be considered in future-generation
communication. Further, a communication system design in which
services/UEs sensitive to reliability and latency are considered is
under discussion. The introduction of future-generation RAT has
been discussed by taking into consideration enhanced mobile
broadband communication, massive MTC, ultra-reliable and
low-latency communication (URLLC), and the like. In current 3GPP, a
study of the future-generation mobile communication system after
EPC is being conducted. In the present invention, the corresponding
technology is referred to as a new RAT (NR) or 5G RAT, for
convenience.
An NR communication system demands that much better performance
than a legacy fourth generation (4G) system be supported in terms
of data rate, capacity, latency, energy consumption, and cost.
Accordingly, the NR system needs to make progress in terms of
bandwidth, spectrum, energy, signaling efficiency, and cost per
bit.
<OFDM Numerology>
The new RAT system uses an OFDM transmission scheme or a similar
transmission scheme. The new RAT system may follow the OFDM
parameters different from OFDM parameters of the LTE system.
Alternatively, the new RAT system may conform to numerology of the
legacy LTE/LTE-A system but may have a broader system bandwidth
(e.g., 100 MHz) than the legacy LTE/LTE-A system. One cell may
support a plurality of numerologies. That is, UEs that operate with
different numerologies may coexist within one cell.
<Slot Structure>
In the 3GPP LTE/LTE-A system, radio frame is 10 ms (307,200
T.sub.s) in duration. The radio frame is divided into 10 subframes
of equal size. Subframe numbers may be assigned to the 10 subframes
within one radio frame, respectively. Here, T.sub.s denotes
sampling time where T.sub.s=1/(2048*15 kHz). Each subframe is 1 ms
long and is further divided into two slots. 20 slots are
sequentially numbered from 0 to 19 in one radio frame. Duration of
each slot is 0.5 ms. A time interval in which one subframe is
transmitted is defined as a transmission time interval (TTI). Time
resources may be distinguished by a radio frame number (or radio
frame index), a subframe number (or subframe index), a slot number
(or slot index), and the like. The TTI refers to an interval during
which data can be scheduled. For example, in a current LTE/LTE-A
system, a transmission opportunity of a UL grant or a DL grant is
present every 1 ms and several transmission opportunities of the
UL/DL grant are not present within a shorter time than 1 ms.
Therefore, the TTI in the legacy LTE/LTE-A system is 1 ms.
FIG. 3 illustrates a slot structure available in a new radio access
technology (NR).
To minimize data transmission latency, in a 5G new RAT, a slot
structure in which a control channel and a data channel are
time-division-multiplexed is considered.
In FIG. 3, the hatched area represents the transmission region of a
DL control channel (e.g., PDCCH) carrying the DCI, and the black
area represents the transmission region of a UL control channel
(e.g., PUCCH) carrying the UCI. Here, the DCI is control
information that the gNB transmits to the UE. The DCI may include
information on cell configuration that the UE should know, DL
specific information such as DL scheduling, and UL specific
information such as UL grant. The UCI is control information that
the UE transmits to the gNB. The UCI may include a HARQ ACK/NACK
report on the DL data, a CSI report on the DL channel status, and a
scheduling request (SR).
In FIG. 3, the region of symbols from symbol index 1 to symbol
index 12 may be used for transmission of a physical channel (e.g.,
a PDSCH) carrying downlink data, or may be used for transmission of
a physical channel (e.g., PUSCH) carrying uplink data. According to
the slot structure of FIG. 3, DL transmission and UL transmission
may be sequentially performed in one slot, and thus
transmission/reception of DL data and reception/transmission of UL
ACK/NACK for the DL data may be performed in one slot. As a result,
the time taken to retransmit data when a data transmission error
occurs may be reduced, thereby minimizing the latency of final data
transmission.
In such a slot structure, a time gap is needed for the process of
switching from the transmission mode to the reception mode or from
the reception mode to the transmission mode of the gNB and UE. On
behalf of the process of switching between the transmission mode
and the reception mode, some OFDM symbols at the time of switching
from DL to UL in the slot structure are set as a guard period
(GP).
In the legacy LTE/LTE-A system, a DL control channel is
time-division-multiplexed with a data channel and a PDCCH, which is
a control channel, is transmitted throughout an entire system band.
However, in the new RAT, it is expected that a bandwidth of one
system reaches approximately a minimum of 100 MHz and it is
difficult to distribute the control channel throughout the entire
band for transmission of the control channel. For data
transmission/reception of a UE, if the entire band is monitored to
receive the DL control channel, this may cause increase in battery
consumption of the UE and deterioration in efficiency. Accordingly,
in the present invention, the DL control channel may be locally
transmitted or distributively transmitted in a partial frequency
band in a system band, i.e., a channel band.
In the NR system, the basic transmission unit is a slot. A duration
of the slot includes 14 symbols having a normal cyclic prefix (CP)
or 12 symbols having an extended CP. In addition, the slot is
scaled in time as a function of a used subcarrier spacing.
In the NR system, a scheduler allocates a radio resource in a unit
of TTI. In the NR system, TTI may be one mini-slot, one slot, or a
plurality of slots.
<Analog Beamforming>
In the millimeter wave (mmW), the wavelength is shortened, and thus
a plurality of antenna elements may be installed in the same area.
For example, a total of 100 antenna elements may be installed in a
5-by-5 cm panel in a 30 GHz band with a wavelength of about 1 cm in
a 2-dimensional array at intervals of 0.5.lamda. (wavelength).
Therefore, in mmW, increasing the coverage or the throughput by
increasing the beamforming (BF) gain using multiple antenna
elements is taken into consideration.
If a transceiver unit (TXRU) is provided for each antenna element
to enable adjustment of transmit power and phase, independent
beamforming is possible for each frequency resource. However,
installing TXRU in all of the about 100 antenna elements is less
feasible in terms of cost. Therefore, considered is a method where
multiple antenna elements are mapped to one TXRU and a beam
direction is adjusted using an analog phase shifter. This analog
beamforming method may only make one beam direction in the whole
band, and thus may not perform frequency selective beamforming
(BF), which is disadvantageous.
The hybrid BF method can be considered which is an intermediate
type of digital BF and analog BF and uses B TXRUs less in number
than Q antenna elements. In the case of hybrid BF, the number of
directions in which beams may be transmitted at the same time is
limited to B or less, which depends on the method of collection of
B TXRUs and Q antenna elements.
<Hybrid Analog Beamforming>
FIG. 4 abstractly illustrates TXRUs and a hybrid BF structure in
terms of physical antennas.
When a plurality of antennas is used, a hybrid BF method in which
digital BF and analog BF are combined is considered. Analog BF (or
RF BF) refers to an operation in which an RF unit performs
precoding (or combining). In hybrid BF, each of a baseband unit and
the RF unit performs precoding (or combining) so that performance
approximating to digital BF can be obtained while the number of RF
chains and the number of digital-to-analog (D/A) (or
analog-to-digital (A/D)) converters is reduced. For convenience,
the hybrid BF structure may be expressed as N TXRUs and M physical
antennas. Digital BF for L data layers to be transmitted by a
transmitter may be expressed as an N-by-L matrix. Next, N converted
digital signals are converted into analog signals through the TXRUs
and analog BF expressed as an M-by-N matrix is applied to the
analog signals. In FIG. 4, the number of digital beams is L and the
number of analog beams is N. In the NR system, the BS is designed
so as to change analog BF in units of symbols and efficient BF
support for a UE located in a specific region is considered. If the
N TXRUs and the M RF antennas are defined as one antenna panel, the
NR system considers even a method of introducing plural antenna
panels to which independent hybrid BF is applicable. In this way,
when the BS uses a plurality of analog beams, since which analog
beam is favorable for signal reception may differ according to each
UE, a beam sweeping operation is considered so that, for at least a
synchronization signal, system information, and paging, all UEs may
have reception opportunities by changing a plurality of analog
beams, that the BS is to apply, according to symbols in a specific
slot or subframe.
Recently, a 3GPP standardization organization is considering
network slicing to achieve a plurality of logical networks in a
single physical network in a new RAT system, i.e., the NR system,
which is a 5G wireless communication system. The logical networks
should be capable of supporting various services (e.g., eMBB, mMTC,
URLLC, etc.) having various requirements. A physical layer system
of the NR system considers a method supporting an orthogonal
frequency division multiplexing (OFDM) scheme using variable
numerologies according to various services. In other words, the NR
system may consider the OFDM scheme (or multiple access scheme)
using independent numerologies in respective time and frequency
resource regions.
Recently, as data traffic remarkably increases with appearance of
smartphone devices, the NR system needs to support of higher
communication capacity (e.g., data throughput). One method
considered to raise the communication capacity is to transmit data
using a plurality of transmission (or reception) antennas. If
digital BF is desired to be applied to the multiple antennas, each
antenna requires an RF chain (e.g., a chain consisting of RF
elements such as a power amplifier and a down converter) and a D/A
or A/D converter. This structure increases hardware complexity and
consumes high power which may not be practical. Accordingly, when
multiple antennas are used, the NR system considers the
above-mentioned hybrid BF method in which digital BF and analog BF
are combined.
FIG. 5 illustrates a cell of a new radio access technology (NR)
system.
Referring to FIG. 5, in the NR system, a method in which a
plurality of transmission and reception points (TRPs) form one cell
is being discussed unlike a wireless communication system of legacy
LTE in which one BS forms one cell. If the plural TRPs form one
cell, seamless communication can be provided even when a TRP that
provides a service to a UE is changed so that mobility management
of the UE is facilitated.
In an LTE/LTE-A system, a PSS/SSS is transmitted
omni-directionally. Meanwhile, a method is considered in which a
gNB which uses millimeter wave (mmWave) transmits a signal such as
a PSS/SSS/PBCH through BF while sweeping beam directions
omni-directionally. Transmission/reception of a signal while
sweeping beam directions is referred to as beam sweeping or beam
scanning. In the present invention, "beam sweeping" represents a
behavior of a transmitter and "beam scanning" represents a behavior
of a receiver. For example, assuming that the gNB may have a
maximum of N beam directions, the gNB transmits a signal such as a
PSS/SSS/PBCH in each of the N beam directions. That is, the gNB
transmits a synchronization signal such as the PSS/SSS/PBCH in each
direction while sweeping directions that the gNB can have or the
gNB desires to support. Alternatively, when the gNB can form N
beams, one beam group may be configured by grouping a few beams and
the PSS/SSS/PBCH may be transmitted/received with respect to each
beam group. In this case, one beam group includes one or more
beams. The signal such as the PSS/SSS/PBCH transmitted in the same
direction may be defined as one synchronization (SS) block and a
plurality of SS blocks may be present in one cell. When the plural
SS blocks are present, SS block indexes may be used to distinguish
between the SS blocks. For example, if the PSS/SSS/PBCH is
transmitted in 10 beam directions in one system, the PSS/SSS/PBCH
transmitted in the same direction may constitute one SS block and
it may be understood that 10 SS blocks are present in the system.
In the present invention, a beam index may be interpreted as an SS
block index.
In a multi-beam environment, whether a UE and/or a TRP can
accurately determine a transmission (Tx) or reception (Rx) beam
direction between the UE and the TRP is problematic. In the
multi-beam environment, signal transmission repetition or beam
sweeping for signal reception may be considered according to a
Tx/Rx reciprocal capability of the TRP (e.g., eNB) or the UE. The
Tx/Rx reciprocal capability is also referred to as Tx/Rx beam
correspondence (BC) in the TRP and the UE. In the multi-beam
environment, if the Tx/Rx reciprocal capability in the TRP or the
UE does not hold, the UE may not transmit a UL signal in a beam
direction in which the UE has received a DL signal because an
optimal path of UL may be different from an optimal path of DL.
Tx/Rx BC in the TRP holds, if the TRP can determine a TRP Rx beam
for UL reception based on DL measurement of UE for one or more Tx
beams of the TRP and/or if the TRP can determine a TRP Tx beam for
DL transmission based on UL measurement for one or more Rx beams of
the TRP. Tx/Rx BC in the UE holds if the UE can determine a UE Rx
beam for UL transmission based on DL measurement of UE for one or
more Rx beams of the UE and/or if the UE can determine a UE Tx beam
for DL reception according to indication of the TRP based on UL
measurement for one or more Tx beams of the UE.
RACH resources are associated with a DL broadcast signal, and are
associated with DL transmission (Tx) beam direction in multi-beam
environment. Likewise, in the multi-beam environment, the RACH
resources are associated with a specific SS block index. In this
case, a RACH resource denotes a time/frequency resource in which a
RACH preamble may be transmitted. The RACH resources may be
indexed. In the present invention, even though RACH preamble
transmission and RACH preamble retransmission are performed in a
physical time domain at different PRACH occasions, if RACH preamble
transmission and RACH preamble retransmission are performed using
RACH resources having the same RACH resource index, the RACH
preamble transmission and RACH preamble retransmission may be
considered as RACH preamble transmission/retransmission using the
same RACH resource. In other words, a RACH resource associated with
the same SS block corresponds to a RACH occasion, at which the UE
may transmit PRACH, in view of a time domain, wherein the RACH
occasion may occur periodically in the time domain.
A method for transmitting RACH Msg1 when Tx/Rx beam correspondence
(hereinafter, referred to as BC) of the UE is not hold should be
different from a method for transmitting RACH Ms1 when TxRx beam
correspondence is hold. Considering this, the present invention
suggests a method for controlling a power of RACH Msg1.
Particularly, the present invention suggests a method for
controlling PRACH transmission power during PRACH retransmission
and a random access method considering Tx/Rx BC of UE and TRP in a
multi-beam environment of the NR system.
Hereinafter, in a multi-beam environment where a plurality of beams
are used between a gNB and a UE, an initial access method,
particularly a random access method, which is different from an
initial access method of the legacy communication system due to
features of analog beamforming, will be described, and the UE and
gNB operation according to the present invention and signaling
information/method, which should be transmitted between the UE and
the gNB will be described.
If BC of the UE and the gNB are hold, a transmission power during
RACH preamble retransmission may be determined similarly to the
legacy LTE/LTE-A. That is, the UE increases an actual transmission
power, that is, target received power, as much as a certain level
by increasing a counter for power ramping as much as 1 during every
retransmission. However, if BC of the UE is not hold, even though
the UE transmits RACH for a specific DL beam received at high
quality, since the UE cannot specify a beam direction in uplink
exactly, the UE may be required to transmit RACH preamble in a
plurality of Tx beam directions that the UE may attempt. Since the
UE has no BC capability or is lack of BC capability, the UE
transmits a RACH preamble in several beam directions. However, if
the UE may transmit a RACH preamble in consecutive RACH (time)
resources while sweeping its Tx beam direction, the UE may quickly
determine its Tx beam direction. However, in this case, since the
network should allocate resources to a corresponding beam direction
for a certain time due to features of analog beamforming, network
resources may be used inefficiently. Moreover, the UE having BC
capability does not need such a resource. Therefore, it is
preferable that the UE may transmit a RACH preamble in one specific
direction during every RACH preamble transmission. However, in this
case, the UE having no BC capability may be required to transmit a
RACH preamble several times until its Tx beam is determined. This
results in initial access latency of UEs having no BC capability.
This initial access latency may be reduced for a certain level by
inheriting a transmission power value used for previous
transmission without initializing a transmission power if the UE
intends to change its Tx beam direction to a direction different
from that of previous transmission when retransmitting a RACH
preamble during the RACH procedure. The UE may increase the
transmission power value by increasing a power ramping counter
during retransmission for the same Tx beam. If RACH power is
controlled using this method, a transmission counter should be set
separately such that the UE may determine whether to end a RACH
procedure by calculating the number of RACH preamble
retransmissions.
If a UE maintains a power ramping counter value of previous
transmission while changing Tx beams, there may be some problems.
Hereinafter, these problems will be described with reference to
FIG. 6.
FIG. 6 illustrates problems that may occur when a UE maintains a
power ramping counter while changing Tx beams for transmitting RACH
preamble(s).
Referring to FIG. 6(a), in case of a UE which maintains a power
ramping counter when retransmitting a RACH preamble by changing Tx
beams, if the UE continuously changes the Tx beams in a round-robin
algorithm, power ramping cannot occur.
Also, referring to FIG. 6(b), the UE may selfishly operate to
reserve a contention priority in a contention based random access
procedure. That is, the UE may first perform power ramping in a
specific beam selected during retransmission and change Tx beam
after sufficiently increasing a transmission power, whereby the UE
may transmit RACH preamble at a maximum transmission power (or very
high target received power), as the case may be, for a Tx beam
direction which is not attempted once, during the corresponding
RACH procedure. In other words, the UE may perform RACH preamble
transmission at a maximum transmission power even though the UE has
not transmitted a RACH preamble in all Tx beams within the number
of maximum transmissions.
FIG. 7 illustrates a beam switching method for RACH preamble
transmission/retransmission.
To solve problems described in FIG. 7, that is, to control
neighboring cell/UE interference with a normal operation of the
RACH procedure, a proper UE beam switching rule should be defined.
For example, referring to FIG. 7, it is assumed that there are
three Tx beams of the UE, that is, the UE may perform transmissions
in three beam directions. In this case, a UE beam sweeping rule may
be determined such that the UE may first perform beam switching
when retransmitting a RACH preamble and then perform power ramping.
In this case, as illustrated in FIG. 7, after the UE attempts all
of its beams, the UE performs power ramping by using a Tx beam,
which has been used last, once again. The UE transmits a RACH
preamble with the same Tx beam as a previous Tx beam for power
ramping without performing beam switching, only after transmitting
RACH preambles for its all beams, that is, only after performing
beam switching first. However, since the number of Tx beams that
the UE has is different per UE and the network does not know the
number of Tx beams in advance, it is not proper that this operation
is forced to the UE.
For this reason, constraints for Tx beam change and power ramping
of the UE should be given for RACH preamble retransmission, whereby
it should be prevented that a random operation of the UE causes
unnecessary interference with respect to the system. The present
invention suggests a method for efficiently (re)transmitting a RACH
preamble by a UE having no BC capability or having partial BC
capability while avoiding the random operation of the UE.
Before transmitting a RACH preamble, the UE should determine the
number of beam directions for which the UE will attempt the RACH
procedure. This is different from that the UE receives a plurality
of signals (e.g., SS blocks) transmitted from gNB through DL and
determines the number of SS blocks for which the UE will perform
the RACH procedure. This relates to how many Tx beams the UE uses
when attempting RACH preamble transmission and which direction the
UE should attempt the RACH preamble transmission for, when the UE
selects one SS block and transmits the RACH preamble for the
selected SS block. Before the UE transmits a RACH preamble, a
negotiation as to how many Tx beams should be used by the UE to
transmit RACH preamble should be made between a higher layer (at
least layer 2) and a layer 1 (that is, physical layer) of the UE.
In case of a UE having BC capability, one Tx beam direction may be
sufficient. In this case, a higher layer of the corresponding UE
notifies the layer 1 that the number of Tx beams, which may be used
for RACH preamble transmission for SS block, is 1. In case of a UE
having no BC capability, a plurality of Tx beam directions should
be given, and the higher layer of the UE informs the layer 1 of the
number of Tx beams, that is, the number of Tx beam directions. The
number of Tx beams, which may be used for RACH preamble
transmission for each SS block, may be 2 to dozens.
The Tx beam set should be negotiated between the layer 1 and the
layer 2 within the UE, and may be different per UE, and the number
of beams within the Tx beam set is related to BC capability of the
UE. If the UE determines the best SS block or preferred SS block
for the RACH (procedure), the UE needs to determine its Tx beam
direction. If the UE does not have BC capability, the UE needs to
attempt several Tx beam directions for a target SS block. In the
present invention, the Tx beam set means beams with which the UE
may attempt a RACH transmission for targeting SS block. The Tx beam
set is determined based on the SS block, and if the UE has a
complete BC capability, only one Tx beam may exist within the Tx
beam set. The number of beams within the Tx beam set may be
different depending on a level of BC that the UE has, and if BC
performance becomes worse, more beams may be used per SS block.
Therefore, a negotiation within L1 and L2 should be made for the Tx
beam set and beam information within the Tx beam set. L2 should
give L1 the number of Tx beams and beam direction information
(e.g., weight vector, spatial parameters, etc.) for the selected
RACH resource.
A method for constraining a transmission power according to beam
switching suggested in the present invention is not applied to a UE
of which BC is hold (e.g., the number of beams is 1), and is
applied to only a case of a UE of which BC is not hold.
Alternatively, application of the present invention may be
determined depending on the number of Tx beams in the network. For
example, if the number of beams within the Tx beam set is N.sub.tx
or less, constraints of a transmission power according to beam
direction change suggested in the present invention may not be
applied, and the constraints may be applied only if the number of
beams within the Tx beam set exceeds N.sub.tx. N.sub.tx may be
configured by the network and signaled to the UE. Hereinafter, the
suggestions of the present invention, which are used to control or
determine RACH preamble transmission power, will be described in
detail.
Suggestion 1) Constraint of Transmission Power Per Tx Beam
PRACH transmission/retransmission procedure may be described as
follows.
A transmission power for transmitting RACH preamble in a physical
layer of the UE is determined by the following Equation.
P.sub.PRACH=min{P.sub.CMAX,c(i),PREAMBLE_RECEIVED_TARGET_POWER+PL.sub.c}_-
[dBm]. Equation (1):
In Equation (1), P.sub.CMAX,c(i) is the configured UE transmit
power for slot i of serving c and PL.sub.c is downlink path loss
estimate calculated in the UE for serving c.
In Equation (1), PREAMBLE_RECEIVED_TARGET_POWER is a value
indicated by a higher layer (e.g. layer 2), the value of
PREAMBLE_RECEIVED_TARGET_POWER in the higher layer is determined by
setting PREAMBLE_RECEIVED_TARGET_POWER to the following equation.
preambleInitialReceivedTargetPower+DELTA_PREAMBLE+(POWER_RAMPING_COUNTER--
1)*powerRampingStep. Equation (2):
In the Equation (2), values of preambleInitialReceivedTargetPower,
DELTA_PREAMBLE, and powerRampingStep are configured to the UE by
network signaling in advance. If the UE initiates a RACH procedure,
POWER_RAMPING_COUNTER is initialized to a specific value, for
example, POWER_RAMPING_COUNTER=1. To calculate the number of
preamble transmissions at the UE, PREAMBLE_TRANSMISSION_COUNTER may
be configured separately, and PREAMBLE_TRANSMISSION_COUNTER is also
initialized to a certain value, for example, 1. If the UE
determines that it has not received RAR successfully after
transmitting a RACH preamble, the physical layer delivers
information indicating that RAR has not been received successfully,
to the higher layer. If this information is received, the higher
layer may instruct the physical layer to attempt RACH
retransmission. In other words, the UE which has not received RAR
successfully after transmitting a RACH preamble may attempt PRACH
retransmission. If RAR is not received successfully, first the UE
increases PREAMBLE_TRANSMISSION_COUNTER as much as 1, and
identifies whether the corresponding RACH preamble transmission is
within the configured maximum number of retransmissions. If RAR is
not received successfully even after the UE has attempted the RACH
preamble transmission up to the maximum number of retransmissions,
the UE ends the RACH procedure and reports to the higher layer that
the RACH procedure has been failed. In other words, if RAR is not
received within RAR window, the UE or layer 2 of the UE: increment
PREAMBLE_TRANSMISSION_COUNTER by 1; If
PREAMBLE_TRANSMISSION_COUNTER=preambleTransMax+1: indicate a random
access problem to upper layers.
However, if the number of RACH preamble transmissions up to the
previous transmission is smaller than the maximum number of
retransmission allowable (e.g., preambleTransMax), the UE may
attempt retransmission of RACH preamble, and first determines
whether to change or maintain a Tx beam direction for
retransmission of the RACH preamble. As described above, in order
that the UE first attempts power ramping by using a specific beam
to prevent excessive interference from being caused, the UE which
may perform Tx beam switching may be constrained so as not to use
one beam consecutively M times or more. In this case, if the UE
intends to transmit a PRACH by selecting a beam direction different
from a beam direction used for the previous PRACH transmission, the
UE may select a beam without separate constraint. However, if the
UE intends to transmit PRACH by selecting a beam direction which is
the same as a beam direction used for previous PRACH transmission,
the UE may calculate the number of consecutive PRACH transmissions
with the corresponding beam. As a result, if the number of
consecutive PRACH transmissions is equal to M, the UE should
transmit a PRACH with another beam. If the UE transmits a PRACH in
a specific beam direction by exceeding M times, the UE uses another
beam direction other than the corresponding beam during later PRACH
retransmission. In other words, if RAR is not received within RAR
window, the UE or layer 2 of the UE: increments
consecutive_transmission_counter [k] by 1, where k is a Tx beam
index of the UE, which is previously used for RACH preamble
transmission. If beam index selected for next RACH preamble=n, if
n=k, if consecutive_transmission_counter [k]=M, a beam index
n(.noteq.k) is reselected; else, increments POWER_RAMPING_COUNTER
by 1, else, maintains POWER_RAMPING_COUNTER.
If the selected beam index `n` is different from `k`, a value for a
previous transmission is inherited as a value of
POWER_RAMPING_COUNTER, and if the selected beam index `n` is equal
to the beam index `k` used for previous transmission,
POWER_RAMPING_COUNTER is incremented by 1.
Afterwards, although PRACH transmission power of the UE may be
determined as expressed in the Equation (1), additional constraints
may be defined in determining a transmission power. For example,
let's assume that the UE attempts power ramping of several times by
transmitting a RACH preamble in a beam direction and thus
PREAMBLE_RECEIVED_TARGET_POWER of the UE is set to a relatively
high value. However, if the UE transmits a RACH preamble at
PREAMBLE_RECEIVED_TARGET_POWER which is relatively high with
respect to a beam direction, which has not been attempted until
now, during RACH preamble retransmission, the PRACH transmission
may cause high inter-cell interference and/or high intra-cell
interference. Therefore, to solve this problem, the present
invention suggests the following methods.
1) Method A: suggests that the UE should store the most recent
transmission power history per Tx. When the UE intends to change a
Tx beam direction in a state that the power ramping counter of the
UE is increased at some level, a transmission power value in a
corresponding beam direction may be defined based on the history so
as not to be increased at some level (e.g., X dB) or more compared
with the most recent transmission power at which the has
transmitted a RACH preamble with the corresponding beam. For
example, PRACH transmission power P.sub.PRACH may be determined as
expressed by the following Equation:
P.sub.PRACH=min{P.sub.CMAX,c(i), PREAMBLE_RECEIVED_TARGET_POWER+PL,
P.sub.PRACH,j[k]+X}_[dBm], where P.sub.PRACH,j[k] is a transmission
power of a RACH preamble using a Tx beam index kin the j-th slot,
and j indicates a previous timing prior to a timing point for
determining P.sub.PRACH. An exact value for X may previously be
signaled to the UE by the network.
2) Method B: is a modified method of the method A, and may be
defined such that a certain constraint according to the present
invention is applied during transmission power determination only
if a specific condition is satisfied. For example, transmission
power determination of the UE based on the aforementioned Equation
and procedure may be applied together with the following additional
conditions and constraints. Conditions Condition i.
PREAMBLE_RECEIVED_TARGET_POWER+PL .gtoreq.P.sub.CMAX,c(i). That is,
if PREAMBLE_RECEIVED_TARGET_POWER+PL value calculated by the higher
layer exceeds an uplink maximum transmission power P.sub.CMAX,c(i),
Condition ii. If the transmission power value indicated by the
higher layer in accordance with RACH preamble retransmission and
power ramping exceeds a power level P.sub.set configured by gNB
(that is, PREAMBLE_RECEIVED_TARGET_POWER+PL .gtoreq.P.sub.set,
wherein P.sub.set is configured by the network), Condition iii. If
PREAMBLE_RECEIVED_TARGET_POWER value calculated based on RACH
preamble retransmission and beam selection exceeds a value
configured by the network (that is, PREAMBLE_RECEIVED_TARGET_POWER
.gtoreq.P.sub.max_preamble_received_target, wherein
P.sub.max_preamble_received_target is configured by the network),
and/or Condition iv. A maximum value of the power ramping counter
may be configured by the network. The power ramping counter is
calculated in accordance with a beam selected for PRACH
retransmission by the UE, and this condition correspond to a case
that the power ramping counter value calculated by the UE exceeds
the power ramping counter maximum value M.sub.max.
The network may select and signal one or combination of the
conditions i to iv as a condition for determining whether to apply
additional constraint when a transmission power of the UE is
determined. If this condition is satisfied, the UE may be
constrained to reduce a transmission power during PRACH
retransmission. If this condition is satisfied, the UE may reset
the transmission power or determine a retransmission power as a
value indicated by the gNB. The additional constraint will be
described as follows.
Constraint (Additional Operation)
FIGS. 8 and 9 illustrate PRACH transmission/retransmission and
corresponding PRACH transmission power according to the present
invention.
The present invention suggests that the UE should transmit RACH
preamble at a corresponding power (e.g., maximum transmission
power) as much as a certain number of times and then reset PRACH
transmission power to an initial value P.sub.init (or a specific
transmission power designated by the network) if the condition i,
the condition ii, and the condition iii and/or the condition iv are
satisfied, for example, referring to FIGS. 8 and 9, if the UE
selects and transmits a RACH preamble after the condition i is
satisfied, that is, the transmission power calculated by the UE
reaches (or exceeds) a maximum transmission power. In this case,
the number of times that the UE may transmit RACH preamble(s) at a
maximum transmission power P.sub.max may be equal to the number of
Tx beams of the UE, for example. The UE may freely select its Tx
beam for the number of times that the UE is allowed to transmit
RACH preamble(s) at a maximum transmission power, before resetting
the transmission power. Afterwards, the UE performs power ramping
in accordance with the number of RACH preamble retransmission and a
beam direction, starting from the reset value.
The condition ii is a modified condition of the condition i, and if
the PRACH transmission power calculated by the UE reaches (or
exceeds) a specific transmission power value P.sub.set configured
by the network and the UE transmits a RACH preamble by selecting
different Tx beams, the UE transmits PRACH (that is, RACH preamble)
at the corresponding power P.sub.set as much as the certain number
of times and then resets the PRACH transmission power to an initial
value or a value configured by the network during PRACH
retransmission. The number of times that the UE is allowed to
transmit RACH preamble at a transmission power P.sub.set may be
restricted, and the maximum number of times that the UE is allowed
to transmit RACH preamble at a transmission power P.sub.set may be
equal to the number of beams of the UE or may be configured by the
network. That is, the UE may perform RACH preamble retransmission
by selecting different Tx beams as much as the configured number of
times. Afterwards, the UE performs power ramping in accordance with
RACH preamble retransmission times and beam direction, starting
from the reset value.
For the condition iii, the condition is configured such that
PREAMBLE_RECEIVED_TARGET_POWER reaches (or exceeds) a specific
value to facilitate the operation in L2, since the PRACH
transmission power is determined in layer 1 (that is, L1) and beam
selection and ramping/transmission counter are determined in layer
2 (that is, L2). If PREAMBLE_RECEIVED_TARGET_POWER reaches (or
exceeds) a specific value, later operation is similar to the
aforementioned operation. That is, in case of
PREAMBLE_RECEIVED_TARGET_POWER .gtoreq.P.sub.max_preamble_received
target,
PREAMBLE_RECEIVED_TARGET_POWER=P.sub.max_preamble_received_target i
is set, whereby PRACH transmission power is determined. For
example, the PRACH transmission power P.sub.PRACH is determined as
P.sub.PRACH=min{P.sub.CMAX,c(i),
P.sub.max_preamble_received_target+PL} [dBm]. If the UE intends to
transmit PRACH by changing Tx beam, the UE may transmit RACH
preamble at the corresponding transmission power up to L times. In
other words, the UE may transmit PRACH at the corresponding
transmission power by using (maximum) L Tx beams. After the number
of PRACH transmission exceeds L times, the PRACH transmission power
is determined by a specific value configured by the network. L may
be equal to the number of Tx beams of the UE or may be signaled to
the UE by the network. Afterwards, if the UE attempts
retransmission of RACH preamble, PREAMBLE_RECEIVED_TARGET_POWER of
the UE may be reset to an initial value or a value configured by
the network.
Referring to FIG. 9, if the transmission power is constrained by
the condition i, the condition ii, or the condition iii, the UE may
retransmit RACH preamble as much as the allowed number of times at
P.sub.CMAX,c(i) or the power value configured by the network, and
then may reset the transmission power to the initialized value
P.sub.init to perform power ramping. Alternatively, the
retransmission power may be designated to a value indicated by the
eNB not the initial value.
If the transmission power is constrained by the condition iv and
the condition iv is satisfied, the power ramping counter of the UE
may be defined so as not to be increased any more. Alternatively,
if the corresponding condition is satisfied, the power ramping
counter of the UE may be reset to the initial value (e.g., 1) or a
specific value configured by the network during later PRACH
retransmission.
In respect of the aforementioned operation(s), the gNB may
configure a plurality of different conditions. For example, the gNB
may configure a plurality of power levels, and the number of times
that a UE is allowed to transmit RACH preamble may be designated
differently per corresponding power level for a case that RACH
preamble transmission power (or calculated
PREAMBLE_RECEIVED_TARGET_POWER value) reaches each power level. For
example, it is assumed that power levels P1 and P2(P1<P2) are
designated, and the number of times that a UE is allowed to
transmit RACH preamble at P1 when the transmission power reaches P1
is designated as N1 times and the number of times that a UE is
allowed to transmit RACH preamble at P2 when the transmission power
reaches P2 is designated as N2 times. In this case, if the RACH
preamble transmission power (or calculated
PREAMBLE_RECEIVED_TARGET_POWER value) reaches P1, the UE may
retransmit RACH preamble at the transmission power P1 maximum N1
times. Retransmission of the RACH preamble at the same transmission
power P1 N1 times in spite of increase of the retransmission times
means that the UE may transmit RACH preamble maximum N1 times while
changing Tx beams. That is, this means that the number of Tx beams
through which the UE may attempt RACH preamble transmission at the
transmission power P1 is limited to N1. If the RACH procedure is
not performed successfully and thus additional retransmission is
performed even after RACH retransmission is performed as much as N1
times, power ramping should be performed for the PRACH transmission
power during (N1+1)-th retransmission. The UE may perform
retransmission of maximum N1 times at a newly updated power. If the
transmission power (or PREAMBLE_RECEIVED_TARGET_POWER) reaches P1,
the number of Tx beams through which the UE may transmit RACH
preamble is limited to N1 per power level greater than P1. Power
ramping during later PRACH retransmission depends on a general RACH
power control except constraint that the number of Tx beams is N1.
That is, although a transmission power is ramping up every PRACH
retransmission, the power ramping counter is maintained as it is
when the Tx beam is changed. In this way, although the UE has
performed PRACH retransmission and power ramping, if the
transmission power (or calculated PREAMBLE_RECEIVED_TARGET_POWER)
reaches P2, the UE may retransmit RACH preamble at a power of P2
(or calculated PREAMBLE_RECEIVED_TARGET_POWER) until N2 times. That
is, RACH preamble may be transmitted in N2 Tx beams only.
PREAMBLE_RECEIVED_TARGET_POWER will be described as an example: If
PREAMBLE_RECEIVED_TARGET_POWER <P1, power ramping for PRACH
transmission power is performed without separate constraint during
PRACH retransmission. If P1.ltoreq.PREAMBLE_RECEIVED_TARGET_POWER
<P2, the number of beams that may be used by the UE during RACH
preamble transmission is limited to N1. That is, the number of
times that the UE may transmit RACH preamble at the same power is
limited to N1. The number of times that the UE may transmit RACH
preamble in a state that the UE does not change the power ramping
counter is limited to N1. If
P2.ltoreq.PREAMBLE_RECEIVED_TARGET_POWER, the number of beams that
may be used by the UE during RACH preamble transmission is limited
to N2. That is, the number of times that the UE may transmit RACH
preamble at the same power is limited to N1. The number of times
that the UE may transmit RACH preamble in a state that the UE does
not change the power ramping counter is limited to N2.
In case of the condition iv, a plurality of values are configured,
and the power ramping counter may be reset whenever the power
ramping counter reaches a value among the plurality of values.
Alternatively, a plurality of values are configured (to the UE by
the network) with respect to the power ramping counter, and the
number of times that the UE may transmit RACH preamble in a state
that the corresponding power ramping counter is maintained may be
designated (by the network) whenever each condition is satisfied.
For example, it is assumed that RACH preamble may be transmitted N1
times when the power ramping counter reaches PC1 and may be
transmitted N2 times when the power ramping counter reaches PC2.
PC1, PC2, N1 and N2 are values configured to the UE by the network.
In this case: If the power ramping counter <PC1, a separate
constraint is not given to power ramping or beam switching during
PRACH retransmission. A general power control rule is applied. If
PC1.ltoreq.power ramping counter <PC2, the number of beams that
may be attempted by the UE during RACH preamble retransmission is
limited to N1. If PC1.ltoreq.power ramping counter
(.ltoreq.M.sub.max), the number of beams that may be attempted by
the UE during RACH preamble retransmission is limited to N2.
The condition(s) and constraint by the corresponding condition(s)
are applied if the UE changes Tx beams during later RACH preamble
retransmission. This constraint is not limited if the UE does not
change Tx beams. That is, even though the RACH preamble
transmission power of the UE reaches a specific power level, if the
UE does not change Tx beams during later RACH preamble
transmission, the UE may perform power ramping continuously, and
has no reason to transmit RACH preamble by reducing (or
maintaining) the transmission power.
Whether to apply the condition(s) and the constraint by the
corresponding condition(s) is determined by whether there is
capability that can determine correspondence to Tx/Rx beam
direction of the UE. Before transmitting RACH preamble, the UE
should determine how many beam directions exist for which the UE
attempts PRACH. This is different from that the UE receives a
plurality of signals (e.g., SS blocks) transmitted from the gNB
through a DL and determines the number of SS blocks that the UE
will perform the RACH procedure, and relates to how many Tx beams
the UE uses when attempting RACH preamble transmission and which
direction the UE should attempt the RACH preamble transmission,
when selecting one SS block and transmitting RACH preamble for the
selected SS block. Before the UE transmits RACH preamble, a
negotiation as to how many Tx beams should be used by the UE to
transmit RACH preamble should be made between a higher layer (at
least layer 2) and a layer 1 of the UE. In case of a UE having BC
capability, one Tx beam direction may be sufficient. In this case,
a higher layer of the corresponding UE notifies the layer 1 that
the number of Tx beams is 1. In case of a UE having no BC
capability, a plurality of Tx beam directions should be given, and
the higher layer of the UE notifies the layer 1 of the number of Tx
beams, that is, the number of Tx beam directions. The number of Tx
beams, which may be used for RACH preamble transmission for each SS
block, may be 2 to dozens.
The method for constraining a transmission power according to beam
switching that the present invention suggests is not applied to a
UE of which BC is hold (e.g., the number of Tx beams is 1), and is
applied to only a case of a UE of which BC is not hold.
Alternatively, whether to apply the present invention may be
determined by the network depending on the number of Tx beams. For
example, if the number of beams within the Tx beam set is N.sub.tx
or less, constraints of a transmission power according to beam
direction change which are suggested in the present invention may
not be applied, and the constraints may be applied only if the
number of beams within the Tx beam set exceeds N.sub.tx. N.sub.tx
may be configured by the network and signaled to the UE.
Suggestion 2) Grouping of Tx Beams into One or More Beam Groups and
Configuration/Setting of Power Ramping Counter Per Beam Group
If a plurality of beams exist in the UE, a method for grouping Tx
beams of the UE may be considered. One or more Tx beams may be
allocated to one beam group, and Tx beams which belong to the same
beam group share the power ramping counter. The power ramping
counters between different beam groups are managed independently.
For example, the UE may have maximum Ng beam groups, each of which
has a power ramping counter. Ng power ramping counters for Ng beam
groups are initialized to the same value, wherein Ng is included in
RACH configuration information and transmitted to the UE. The
number Nb of Tx beams per beam group may be selected by the UE. If
the beam groups have the same number of Tx beams, the number of Tx
beams of the UE is Ng*Nb, and beam groups may have different number
of Tx beams depending on panel configuration of the UE.
Preferably, Tx beams included in the same beam group are the beams
of which transmission directions are partially overlapped. If Tx
beams of the UE are grouped into beams of which Tx beam directions
are similar, the UE selects a transmission direction within a
specific beam group and performs power ramping for the selected
beam and then changes a beam direction within the corresponding
beam group. In this case, even though the power ramping counter for
previous PRACH (re)transmission is inherited as it is to transmit
RACH preamble at a ramping-up power, that is, is operated as shown
in FIG. 9(a), serious interference may not be caused in the network
due to similarity of Tx beam directions within the same beam group.
Beams of which Tx beam directions are not similar are grouped into
their respective beam groups different from each other. The beam
groups have their respective power ramping counters, and if the UE
changes the Tx beam directions by changing the beam groups, the
power ramping counter for RACH preamble retransmission is
determined based on the number of retransmissions per beam
group.
If a lot of Tx beams are provided per beam group and transmission
directions between Tx beams within the beam group are different
from each other, the method described in the suggestion 1 as a
method for reducing network interference may equally be applied to
the suggestion 2. That is, the method A or the method B may be
applied to each beam group.
As a modification of this suggestion, if the UE does not have a
separate power ramping counter per beam group and retransmits RACH
preamble by changing Tx beams within the same beam group, the power
ramping counter is increased. However, if the UE changes beams to
another beam group during RACH preamble retransmission, the power
ramping counter may be configured so as not to be changed (that is,
transmission power value or PREAMBLE_RECEIVED_TARGET_POWER is not
changed).
Suggestion 3) Designation of Limitation in RACH Preamble
Retransmission while Power Ramping Counter is not Increased if
Retransmission is Performed by Beam Change
In this suggestion, the basic operation of the UE, "the UE does not
increase the power ramping counter when changing Tx beams" is
performed only if an initial certain condition is satisfied. In
case of a certain threshold or more, even though the UE transmits
RACH preamble by changing beams, the power ramping counter is
increased as much as a certain value (e.g., 1). This is to prevent
initial access latency from being too increased due to too long
delay of the RACH procedure of the UE of which BC is not hold.
It is assumed that "the operation for not increasing the ramping
counter when the UE changes Tx beams during RACH preamble
(re)transmission" is referred to as operation A, and the operation
for always increasing a power ramping counter as much as a certain
value (e.g., 1) during RACH preamble retransmission regardless of
Tx beam change during RACH preamble (re)transmission is referred to
as operation B. A condition for establishing the operation A may
have operations as follows.
Option 1) The UE follows the operation A only in case of power
ramping counter .ltoreq.Npc, and then follows the operation B. In
this case, Npc may be signaled or configured by the network in
advance. Npc is a value that the power ramping counter may have,
and if the power ramping counter is smaller than or equal to Npc,
the UE follows the operation A during RACH preamble retransmission,
but follows the operation B, that is, increases the power ramping
counter regardless of change of Tx beams during RACH preamble
retransmission if the power ramping counter is increased and thus
greater than Npc.
Option 2) The UE performs the operation A only in case of the
number of RACH preamble transmissions .ltoreq.Ncounter, and follows
the operation B in case of the number of RACH preamble
transmissions >Ncounter. Ncounter may be signaled or configured
by the network in advance. The UE does not increase the power
ramping counter when changing Tx beams while the UE performs RACH
preamble transmission Ncounter times. However, if the number of
RACH preamble transmissions becomes greater than Ncounter, the UE
increases the power ramping counter as much as a certain value
(e.g., 1) when changing Tx beams.
Option 3) A value of Nbcounter may be signaled or configured by the
network in advance. The UE may transmit RACH preamble as much as
maximum Nbcounter times in accordance with the operation A. That
is, Nbcounter corresponds to the number of times that a UE
maintains the power ramping counter as it is without increase when
Tx beams are changed. The UE may follow the operation A as much as
maximum Nbcounter times. However, in excess of corresponding times,
the UE increases the power ramping counter if Tx beams are changed
during later RACH preamble retransmission.
Option 4) The UE follows the operation A only in case of
PREAMBLE_RECEIVED_TARGET_POWER .ltoreq.Ptarget_power, and follows
the operation B if a corresponding condition is not satisfied. That
is, if PREAMBLE_RECEIVED_TARGET_POWER calculated by the UE during
RACH preamble retransmission is smaller than or equal to
Ptarget_power, the UE does not increase the power ramping counter
if Tx beams are changed in accordance with the operation A.
However, if PREAMBLE_RECEIVED_TARGET_POWER exceeds Ptarget_power,
the UE increases the power ramping counter as much as a certain
value (e.g., 1) during RACH preamble retransmission regardless of
Tx beams in accordance with the operation B. Ptarget_power may be
signaled or configured by the network in advance.
Option 5) The UE follows the operation A only in case of
P.sub.PRACH.ltoreq.P.sub.level, and follows the operation B if the
corresponding condition is not satisfied. P.sub.level may be
signaled or configured by the network in advance. When a
transmission power of RACH preamble calculated in L1 is smaller
than or equal to P.sub.level, the UE does not increase the power
ramping counter if Tx beams are changed in accordance with the
operation A. However, if the calculated transmission power of the
RACH preamble exceeds P.sub.level, the UE increases the power
ramping counter as much as a certain value (e.g., 1) during RACH
preamble retransmission in accordance with the operation B
regardless of Tx beams.
Suggestion 4) Combination of Suggestion 1 and Suggestion 3
Suggestion 4 is a method for managing interference on a neighboring
UE/cell while reducing latency of RACH procedure in a multi-beam
environment, and corresponds to combination of the suggestion 1 and
the suggestion 3. When the UE of which BC is not hold transmits
RACH preamble, too high interference may be caused in an inexact
direction if the UE first performs power ramping and then performs
beam switching. To avoid this, the number of times for continuously
transmitting RACH preamble in the same beam direction may be
limited. Furthermore, if the UE transmits the RACH preamble prior
to beam switching during initial RACH preamble transmission, power
ramping may be too slow during RACH preamble transmission of the UE
unless the ramping counter is increased unconditionally when Tx
beams are changed. Therefore, a method for not performing power
ramping during beam switching is limited to be applied under a
certain condition, and if the corresponding condition is not
satisfied, the power ramping counter is increased regardless of
beam switching, whereby latency of the RACH procedure may be
prevented from being too increased.
As a modification of the suggestion 4, PRACH power control may be
performed by combination of the suggestions 1, 2 and 3. Even though
a plurality of beams that the UE has are grouped into one or more
beam groups, and RACH preamble is retransmitted while switching the
beams within the same beam group, the power ramping counter is
increased. However, if beam group is changed, the power ramping
counter is maintained. Likewise, to avoid too latency, the
operation for maintaining the power ramping counter may be limited
to be applied under a certain condition (see suggestion 3). Only
when the corresponding condition is maintained, the UE maintains
the power ramping counter if the UE transmits RACH preamble by
changing from a beam belonging to one beam group to a beam
belonging to another beam group. However, when the corresponding
condition is not maintained, the UE increases the power ramping
counter even though the UE transmits the RACH preamble by changing
the beam groups. Details of the corresponding condition are
described in the suggestion 3. If the UE has transmitted the RACH
preamble as much as a certain number of times (or reaches a certain
power level or a certain preamble received target power) by using
only beams within the same beam group, the UE should attempt RACH
preamble transmission by changing the corresponding beam group to
another beam group (see suggestion 1).
Limitation to which Suggested Methods (Suggestion
1.about.Suggestion 4) can be Applied
The basic method (e.g., method for maintaining a power ramping
counter without increase if beams are changed during RACH preamble
retransmission) and the suggestions of the present invention
described to solve the problems derived from the basic method are
actually applied to the UE having a capability capable of
determining correspondence to Tx/Rx beam direction. In case of the
UE having a BC capability, it is preferable to increase the power
ramping counter during every RACH preamble retransmission. However,
it is required to clarify how the UE may specify a BC capability
and clarify a range of beam change. When the UE transmits RACH
preamble, basis of occurrence of beam change may be categorized
into three cases as follows:
1) case that the UE changes Tx beams during RACH preamble
transmission because the UE has no BC capability (for beam
switching);
2) case that the UE changes beams for tracking for DL beams (e.g.,
SS blocks) (for receiving beam tracking); and
3) case that the UE changes receiving beams of gNB by changing DL
beams (e.g., SS blocks) (that is, changing RACH resources) (for
this reason, Tx beam change of the UE may occur).
Among the three cases listed above, the case 3 of RARCH resource
transmission will separately be described below. In this case, two
cases of the case 1 and the case 2 will be described. It is
required to clarify how the UE should perform RACH preamble power
control for the case 1 and the case 2. Tx beam change of the UE
occurs in both two cases. However, the case 2 may frequently occur
in the UE having BC capability due to position/angle change of the
UE.
Before primarily transmitting RACH preamble, the UE should
determine the number of beam directions for which PRACH procedure
is to be attempted. This is different from that the UE receives a
plurality of signals (e.g., SS blocks) transmitted from gNB through
DL and determines the number of SS blocks for which the UE would
perform a RACH procedure. This relates to how many Tx beam the UE
uses when attempting RACH preamble transmission for and which
direction the UE should attempt the RACH preamble transmission,
when the UE selects one SS block and transmits RACH preamble for
the selected SS block. Before the UE transmits RACH preamble, a
negotiation as to how many Tx beams should be used by the UE to
transmit RACH preamble should be made between a higher layer (at
least layer 2) and a layer 1 (that is, physical layer) of the UE.
In case of a UE having a BC capability, one Tx beam direction may
be sufficient. In this case, a higher layer of the corresponding UE
notifies the layer 1 that the number of Tx beam sets is 1. In case
of a UE having no BC capability, a plurality of Tx beam directions
should be given, and the higher layer of the UE notifies the layer
1 of the number of Tx beams, that is, the number of Tx beam
directions. The number of Tx beams, which may be used by the UE
having no BC capability for RACH preamble transmission for each SS
block, may be 2 to dozens. The layer 2 (e.g., L2) provides the
number of Tx beams and beam direction information (e.g., weight
vector, spatial parameters, etc.), which will be attempted by the
UE for RACH preamble transmission in the selected RACH resource, to
the L1.
Basically, if UE Tx beam change occurs in the case 2 due to a
purpose for reception tracking despite that the UE has a BC
capability, it is not preferable to apply a power control method
for maintaining the power ramping counter during beam change. In
this case, the UE should increase the power ramping counter when
performing retransmission by changing beams.
If the higher layer indicates that the number of Tx beams within
the Tx beam set is 1, the UE determines that the UE has a BC
capability and increases the power ramping counter every RACH
preamble retransmission. Even though beam change occurs in the case
2, the UE increases the power ramping counter during
retransmission. If the higher layer (e.g., L2) notifies the lower
layer (e.g., L1) that a plurality of beams are provided in a set of
Tx beams, the UE determines that it has no BC capability and does
not increase the power ramping counter when changing beams during
PRACH retransmission as described in the present invention. The
methods suggested in the present invention may be applied to the
problems that may occur due to the above operation. As another
method, the network may determine whether to apply these
constraints in accordance with the number of Tx beams. For example,
the constraints of the transmission power according to beam
direction change which are suggested in the present invention are
not applied if the number of beams within the Tx beam set is
N.sub.tx or less, whereas the constraints may be applied only if
the number of beams within the Tx beam set exceeds N.sub.tx.
N.sub.tx is set by the network and signaled to the UE.
Alternatively, N.sub.tx beams may allow the UE to assume beams of
which mutual BCs are hold. N.sub.tx may be configured for the UE by
the network, or may be designated in the standard document.
RACH Resource Change
There may be another domain (e.g., RACH resource) for RACH preamble
retransmission in a multi-beam environment. That is, the UE may
switch RACH resources for RACH preamble retransmission. Although
RACH resource selection depends on the UE, some restrictions should
be required for switching of RACH resources to reduce a ping-pong
effect between the RACH resources. In other words, RACH resource
switching for the RACH procedure should depend on a UE having
specific criteria. For example, the UE may switch the RACH
resources if the best received beam (e.g., SS block index) is
changed or multiple beams (e.g., SS blocks) are received in a
similar received quality.
Hereinafter, the RACH resource selection method will be described
in more detail. In this case, the RACH resources refer to
time/frequency resources for transmitting RACH preamble, and may
additionally include a preamble sequence set (or preamble code
set). That is, in a multi-beam environment, if the UE selects a
specific RACH resource for RACH preamble transmission and transmits
the RACH preamble on the specific RACH resource, the specific RACH
resource serves to inform the network of a DL beam direction
preferred by the UE. For example, in a system where a plurality of
SS blocks are transmitted on a cell and are subjected to
beamforming in different DL beam directions, if the UE intends to
attempt RACH for a SS block received with the best quality, the UE
selects a RACH resource associated with the SS block and transmits
a RACH preamble on the RACH resource. The associated relation
between the SS block and the RACH resource may be configured with
an associated relation between a specific SS block and a
time/frequency resource, or with an associated relation between the
SS block and time/frequency/code resource. Additionally, if a
plurality of CSI-RS are configured in the system and an associated
relation with RACH resource per CSI-RS is configured, and if the UE
is configured to transmit RACH in a CSI-RS transmission direction,
the associated relation between CSI-RS and time/frequency/code
resource may be configured. In the present invention, although it
is assumed that the RACH resource is associated with SS block, the
present invention is not only applied to the case that the RACH
resource is associated with the SS block but also applied to the
case that the RACH resource is associated with another DL signal
representative of DL Tx beam direction. For example, in the present
invention, the SS block is a signal/channel representative of DL Tx
beam direction, and may be replaced with another signal/channel
(e.g., CSI-RS) representative of DL Tx beam direction.
PRACH transmission power is determined by the Equation (1) or its
modification, and an open loop power control method for UL
transmission is used for control of a PRACH transmission power. At
this time, PL of the Equation (1) is a downlink path loss and may
be represented by a downlink signal (e.g., SS block) received power
value (e.g., reference signal received power (RSRP) (hereinafter,
SS block RSRP) based on SS block). Also, a preamble received target
power PREAMBLE_RECEIVED_TARGET_POWER of the Equation (1) is a
received power value predicted/expected by the UE when a specific
signal is received in the gNB, and is a UL transmission power value
estimated by the UE. This preamble received target power may be
different from a received power value which actually reaches the
gNB. The preamble received target power is determined by the
initial set value configured by the gNB and a transmission power
increased as much as a certain level whenever the UE fails in RACH
preamble transmission. The UE selects a SS block based on RSRP per
SS block or reference signal. If a plurality of SS blocks are
transmitted on a cell, the UE measures RSRP by assuming that the
same transmission power is applied to each SS block. That is, it is
assumed that the same transmission power is applied to each SS
block unless there is a separate signaling. If the transmission
power is different per SS block, the gNB needs to signal this. For
example, a reference SS block may be designated, and the gNB may
signal a difference in a transmission power per SS block compared
with a transmission power of the reference SS block. In this case,
the reference for selecting SS block, that is, RACH resource in the
UE should not be a simple RSRP, but a value considering a
transmission power ratio of a corresponding SS block in a measured
RSRP.
After the UE transmits the RACH preamble, the UE monitors a DL
control channel (e.g., control resource set (CORSET)) for receiving
RAR for a given time (e.g., RAR window). If RAR is not received
successfully for a corresponding time, the UE attempts RACH
preamble retransmission. When SS block of the best quality is
changed while the UE attempts the RACH preamble retransmission, the
UE should determine whether to use the previous RACH resource used
by the UE as it is or select a new RACH resource for the RACH
preamble retransmission. Whether to maintain the RACH resource or
select new RACH resource, that is, whether to maintain or change a
target SS block is determined depending on when the UE measures SS
block RSRP. A selected RACH resource (that is, selected SS block)
is varied depending on whether to select the RACH resource every
RACH preamble initial transmission timing or select the RACH
resource every RACH preamble (re)transmission timing.
1) RACH Resource Selection Opportunity: During RACH Preamble
Initial Transmission
In this method, the UE selects the RACH resource during RACH
preamble initial transmission. In this method, the UE newly selects
the RACH resource or stops the current RACH procedure to start the
new RACH procedure only if a specific condition configured by the
gNB (or previously defined) is satisfied. That is, the UE measures
SS block RSRP during or just before RACH preamble initial
transmission and transmits a RACH preamble by selecting the RACH
resource based on the SS block RSRP. For convenience of
description, a SS block received with the best quality during or
just before RACH initial transmission or a SS block selected by the
UE is referred to as a target SS. The UE selects a target SS block
and transmits a RACH preamble (that is, Msg1) by using the RACH
resource associated with the target SS Block. In this method, the
UE may change the RACH resource during RACH preamble retransmission
only if a certain condition configured by the gNB or previously
defined is satisfied. Conditions that may change the RACH resource
are as follows.
a) At any time when RACH preamble retransmission is to be
performed, when another SS block having received RSRP better than
the target SS block RSRP at a certain level (e.g., X dB) or more
for a certain time (e.g., T msec) or more is discovered, the UE
changes the SS block to the target SS block and performs RACH
preamble retransmission in the changed RACH resource. In this case,
T and X may be defined in advance, or may be configured for the UE
by the gNB.
b) If the transmission power of the UE reaches a maximum allowed
transmission power due to RACH preamble retransmission of several
times in the RACH resource associated with the target SS block, or
after transmitting the RACH preamble of M times at the maximum
allowed transmission power, the UE changes the RACH resource by
changing the target SS block. In this case, received signal quality
of the new target SS block may be lower than the existing target SS
block RSRP. In this case, M and the maximum allowed transmission
power value may be defined in advance, or may be configured for the
UE by the gNB.
In this method, if the above conditions are satisfied, the UE ends
the RACH procedure using the RACH resource associated with the
existing target SS block. If RACH preamble transmission starts in
the RACH resource associated with the new target SS block, this may
be understood that the new RACH procedure starts. Therefore, RACH
configuration/parameters used for the RACH procedure should be
initialized. If the RACH procedure ends, the physical layer of the
UE delivers RACH procedure failure or termination message to the
higher layer (e.g., L2). If the UE changes the target SS block, the
UE updates the estimated PL value of the Equation (1) to newly
selected target SS block RSRP.
If the UE intends to partially inherit RACH related parameters used
for the existing RACH procedure in starting the new RACH procedure,
it may be understood that the existing RACH procedure does not end
and the following method is performed.
2) RACH Resource Selection Opportunity: Every RACH Preamble
Transmission Timing
In this method, the UE selects the RACH resource during every RACH
preamble (re)transmission. For example, the UE selects a target SS
block by measuring SS block during initial transmission and
transmits a RACH preamble by selecting a RACH resource associated
with the target SS block, and selects a new target SS block when
intending to retransmit the RACH preamble due to failure in RAR
reception and transmits a RACH preamble on the RACH resource
associated with the newly selected target SS block. In this method,
the target SS block may be different per every RACH preamble
retransmission timing. Therefore, RACH preamble retransmission may
be performed in a RACH resource different from the RACH resource of
previous transmission. That is, in this method, there is no
specific restriction in changing the RACH resources, and the UE may
freely attempt RACH preamble transmission for SS Blocks having the
same cell ID. In this method, the UE is not responsible for
performing/constrained to perform measurement for SS block during
every RACH preamble (re)transmission, but may freely change the
RACH resource in accordance with quality of a received signal by
performing measurement for the SS block. If the UE changes the
target SS block, the UE updates the estimated PL value of the
Equation (1) to newly selected target SS block RSRP.
When the target SS block is changed, if the power ramping counter
is initialized, a problem occurs in that latency of the RACH
procedure may be increased. Therefore, in the present invention, if
a UE performs RACH preamble retransmission while changing the RACH
resource, the power ramping counter for determining a received
target power is inherited. Inheritance of the power ramping counter
may be interpreted as follows: the power ramping counter may be
maintained equally to the previous value if the RACH resource is
changed (that is, if the target SS block is changed). If a UE
performs RACH preamble retransmission while changing the RACH
resource, the number of RACH preamble transmissions at the UE are
integrally calculated regardless of change of the target SS
block.
As the UE may select a RACH resource within a full set of the RACH
resources associated with each SS block at a desired time, RACH
preamble retransmission(s) within the resource set of the
corresponding RACH resources is(are) considered as one RACH
procedure. The UE may select a SS block measured with received
quality of a certain level or more or a level similar to that of
the existing target SS block RSRP when selecting a RACH resource.
Criteria for the RACH resource selection are similar to the
conditions `a` and `b` described in the method "1) RACH resource
selection opportunity".
However, maintaining or increasing the RACH preamble power ramping
counter of the UE when retransmitting the RACH preamble by changing
the RACH resource may be unfavorable for the network. That is,
although the UE has retransmitted the RACH preamble while
continuously increasing or maintaining the power ramping counter,
if RSRP value of the SS block, which